Is speedometer relative to odometer?

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  #1  
Old 04-03-10, 09:02 AM
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Is speedometer relative to odometer?

I have owned early 90 Dodge Spirit cars. The last two I had I discovered that the speedometer is off 4 mph at 30 mph, according to police dept. portable radar machines they park along the side of the road. (I am actually going 26 mph.)

I would like to know if the factory did this on purpose so that people might think they are getting better gas mileage than they actually are.

I also want to find out with my new 92 Spirit, if I can look at the odometer and measure a mile, using roadside markers (a tree, a sign, etc.), to see what MPH I am going for that mile. But obviously, this will not work if the odometer and speedometer are tied in with each other.

Too many cars are tailgating me even when my speedometer says 62, and I am really getting upset. I want to know if everyone today thinks they can push the envelope, or if once again, my speedometer is off. And if I have a way(the way I mentioned) of checking myself.

I might add also if you are thinking the tire might have been changed from fatory spec, they have not - on any of the 3 Dodges. And they are also aired up to about the right amount.
 
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Old 04-03-10, 10:18 AM
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The speedometer should be accurate and the odometer should be linked to the speedometer. (Older via gears, newer via electronics.)

And very possible there is a different size tire than factory. Also some newer vehicles can have the tire size entered into the appropriate computer by the dealer. This could be entered wrong. And only a dealer could change that.

It might be possible to have the dealer enter the tire size you are using into the vehicle, then all would be well? But the factory is only going to allow certain tire sizes on a particular vehicle due to safety, braking, height of vehicle/rollover, cornering, etc. (They are not going to have anything to do with getting the vehicle to work with a tire size which might be unsafe.)

As to checking the odometer/speedometer and mile markers... Best to bring a friend along with a watch. Then you can do the driving while the friend does the timing.

For the odometer test, just write down what the mileage is at the 1st mile marker. Then go about 5 miles and see if it is recording 5 miles.

For the speed test, go exactly 60 miles per hour. Then it should be 60 seconds for each mile if accurate. (A mile a minute!)

Or use this along with a stopwatch...
Speedometer Accuracy
 
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Old 04-03-10, 10:44 AM
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Originally Posted by Bill190 View Post
And very possible there is a different size tire than factory. Also some newer vehicles can have the tire size entered into the appropriate computer by the dealer. This could be entered wrong. And only a dealer could change that.
Tire specs written on door jamb. On all 3 vehicles, that is the size I have, on all 4 tires.

As to checking the odometer/speedometer and mile markers... Best to bring a friend along with a watch. Then you can do the driving while the friend does the timing.
What good will that do if the car's odometer, that says a mile, really is not a mile, since we know that the speedometer is wrong, making also the odometer wrong.

I need to call the state DOT and get them to put a mileage marker on my highway, like they have on some other highway in the area, 20 miles from here.
 
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Old 04-03-10, 11:14 AM
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The portable mph signs are just cheap tools to provide an approximate speed of different cars in the width range of the unit. You could be seeing the speed of someone behind you.

They are probably not legal for giving tickets since they are not selective and continuously monitored.

There is nothing better than the mile markers over a 5 or 10 mile stretch that were set by the DOT. There are some on I94 and highway 53, but can always use the green markers set every 0.1 mile apart on most highways, but they may not be as accurate, but over 10 miles they should be good for an average.

Dick
 
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Old 04-03-10, 01:25 PM
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Dick,

You sure about that what you say about the accuracy of the portable radar devices? These look pretty heavy duty and official.....and expensive. And why is it every time I go past one, no matter where it is, at 30 MPH, I am always going 26? I am not that mindless not to know that if I am in a pack of cars, that it might not be me that is being clocked. And then on the highway I get frequently passed when I am going 65(have an inkling I am actually going more like 59) on a 55 mph highway? I really do believe my MPG is off.

Green markers every block? Where? ON the road? Or the side of the road? On 53? On what stretch have you seen these?
 
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Old 04-03-10, 02:19 PM
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Originally Posted by ecman51`
I want to know if everyone today thinks they can push the envelope, or if once again, my speedometer is off. And if I have a way(the way I mentioned) of checking myself.
Everyone today thinks they can push the envelope. And if you're trying to observe the speed limit; you are the envelope.

The most straight forward answer I can suggest beside that, and I really feel that it does, is to borrow or obtain a GPS. On the default "view map" mode, where it's not directing you, it displays your mph. Accurately. Besides they're really cool toys.
Either that find out a cops favorite fishing spot and blow by him on purpose. This would provide you official documentation as well!

I would like to know if the factory did this on purpose so that people might think they are getting better gas mileage than they actually are.
Car companies know that they better not even let a car out the door with the "How to change a tire" instruction sheet missing.
 
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Old 04-03-10, 05:09 PM
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Originally Posted by mickblock View Post
Either that find out a cops favorite fishing spot and blow by him on purpose. This would provide you official documentation as well!
Always entertaining in movies when that happens. American Graffitti(the chained axle scene), Smokey and the Bandit, etc.

The GPS idea is a clever suggestion.
 
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Old 04-03-10, 08:16 PM
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Just be sure to hide it when you are not in the vehicle and clean off the suction cup marks. I am mentioning this because there have been a buch of them stolen out of cars in my area.
 
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Old 04-03-10, 08:45 PM
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Spedo

Speedometers like all the other gauges in a car are not accurate instruments. They are just ment to give you an indication of what is going on. Historicaly they will read slow up to about 50 MPH then read fast from there on. The older police cars had a "certified" spedo that was supposed to be accurate but they werent much better. (I have seen both tested ) As far as people riding your bumper when you are running the speed limit,they will do that at any speed. I was recently on a major highway and wondered how fast I would have to go to keep from being passed. That number was in excess of 90 MPH. If you want to check yours,find a cop with a radar gun,drive past him at an agreed speed and see how they compare.
 
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Old 04-03-10, 08:49 PM
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One other thought just because two tires say the same size on them dont mean they are the same diameter. Measure a few tires of the same size marking you will see what I mean.
 
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Old 04-03-10, 09:05 PM
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I did not say the green markers are on every block.

If you look outside the right shoulder of every freeway (I94 in your case) there will be a marker every 1/10 of a mile apart (except where there is a ramp, bridge or grade separation). They are usually 3"x6" green reflective sign on a post that is usually along the federal/U.S. highways unless the state or local jurisdiction does not choose to participate. In your case, I have seen then on Highway 53 north of I94. You will also see them on Highway 29 east of Highway 53. - You just have to look for them and they are not always continuous, but they are a part of the over-all measurement system that starts at either the west or south beginning of roads from the state boarder (depending on the general E-W or N-S). That system usually identifies the general mile marker of exits in most modern states, but here may be some hold-outs. It has been around for generations.

I don't know what good they will do you if you are trying to fight a speeding ticket, since most police departments offer to follow you and verify your speedometer accuracy in the interest of public safety.

Dick
 
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Old 04-04-10, 04:20 AM
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"I was recently on a major highway and wondered how fast I would have to go to keep from being passed"

A few years ago we went thru Cincinnati on a sunday morning on I-75. I was running 80 mph [speed limit 55] My wife was fussing at me for going slower than the traffic that was continually passing me

With the exception of the occasional driver [going 10-15 mph under the limit] I get stuck behind on these narrow country roads, it's rare to find the majority of traffic going the speed limit. Seems like the majority of drivers run 5-10 mph [or more] above the posted speed limit.
 
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Old 04-04-10, 11:31 AM
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Originally Posted by marksr View Post
"I was recently on a major highway and wondered how fast I would have to go to keep from being passed"

A few years ago we went thru Cincinnati on a sunday morning on I-75. I was running 80 mph [speed limit 55] My wife was fussing at me for going slower than the traffic that was continually passing me

With the exception of the occasional driver [going 10-15 mph under the limit] I get stuck behind on these narrow country roads, it's rare to find the majority of traffic going the speed limit. Seems like the majority of drivers run 5-10 mph [or more] above the posted speed limit.
The police are not doing their job, and have let things get out of control. And now it is almost unfair if a person gets singled out then, and gets a tciket - if "everybody else is doing it".

There is no reason why the speed limit should not mean the speed LIMIT. If they want the limit higher, put up new signs - and if people exceed that, say "that is it! You're getting a ticket!"

What is the difference if they give a fine for 1 mph over the limit, or if they give one at 10, when the person being pulled over would simply say, "I thought I was going 9 over(and maybe that is even what their speedometer said), and since you do not give tickets for 9 over......" See what I mean?

I am a proponent, and think I have brought this up before, that tickets should be incremental, just because of the fact that..... what if the police radar, or the person's speedometer is off say 1-2 mph? It be unfair to let other people slide at 9 over and just because they claim you are 1 more measely mph over, at 10 over, it goes from $0 to $135 or whatever? I think they should have a grace of 3 mph, then start with incremental fines of say $10-15 for every mile over that.
 
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Old 04-04-10, 11:35 AM
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Originally Posted by Concretemasonry View Post
I did not say the green markers are on every block.

If you look outside the right shoulder of every freeway (I94 in your case) there will be a marker every 1/10 of a mile apart (except where there is a ramp, bridge or grade separation). They are usually 3"x6" green reflective sign on a post that is usually along the federal/U.S. highways unless the state or local jurisdiction does not choose to participate. In your case, I have seen then on Highway 53 north of I94. You will also see them on Highway 29 east of Highway 53. - You just have to look for them and they are not always continuous, but they are a part of the over-all measurement system that starts at either the west or south beginning of roads from the state boarder (depending on the general E-W or N-S). That system usually identifies the general mile marker of exits in most modern states, but here may be some hold-outs. It has been around for generations.

I don't know what good they will do you if you are trying to fight a speeding ticket, since most police departments offer to follow you and verify your speedometer accuracy in the interest of public safety.

Dick
I'll have to check that out, Dick, after I leave my hermit existance and brave the high Marksr speeds on the interstate, that I have not been on in years. I'll probably be holding the wheel with a death grip. I'll check out 53 perhaps north of Eau Claire-Hallie, some.
 
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Old 04-04-10, 11:42 AM
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Originally Posted by retired wrench View Post
Speedometers like all the other gauges in a car are not accurate instruments. They are just ment to give you an indication of what is going on.
Seriously? I wonder then how so many drivers dare to press the limit, then if they are not certain if their speedometers are accurate. If I ever get pulled over, I guess then I will tell the cop to send the ticket to Chrysler then.

The last time I got a speeding ticket was for going 62 (not 65) in a 55 zone - 25 years ago. But I have been pulled over for all KINDS of stuff, including not using my directional when the only person that happened to be at the intersection was the cop. Or when my license plate light went out. Or doing donuts. Or illegal u-turn. Or when a cop in town pulled me over for I think 5 misc. rinky dink infractions at once, just to give him something to do.
 
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Old 04-04-10, 11:46 AM
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Originally Posted by retired wrench View Post
One other thought just because two tires say the same size on them dont mean they are the same diameter. Measure a few tires of the same size marking you will see what I mean.
Seriously(again)? Gee, what if a person then had posi-trac and one tire had a bigger circumference than another like-marked tire? You sure about this? Why do they bother labeling tire sizes then?
 
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Old 04-04-10, 11:51 AM
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One policeman told me to keep from getting pulled over, keep it less than 15 over the limit i.e. 49 in a 35 is okay but not 50.
 
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Old 04-04-10, 03:50 PM
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Originally Posted by NEsportsfan View Post
One policeman told me to keep from getting pulled over, keep it less than 15 over the limit i.e. 49 in a 35 is okay but not 50.
Huh. I think they let things get away from them now, way to long.
 
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Old 04-04-10, 03:56 PM
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Thanks Dick!

The sign posts are indeed out there on 53. Vertical blue signs every 2 blocks. I made 3 1-mile runs a bit ago. I did a mile in 103.5 sec. avg., and this calculates out to between 56-7 mph. I used my cell phone stop watch. I did not even have to take my eyes off the road. Did it by feel. The odometer was close enough to exact, as much as I could tell. Yes, if I were to do a 5 mile run, it be more accurate. People sure do go speedin' right along.

Thanks again, as I would have never known about the signs, unless say I had called the trooper headquarters there on 53.
 
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Old 04-05-10, 05:05 AM
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"I am a proponent, and think I have brought this up before, that tickets should be incremental"

If I remember correctly, 20 yrs ago when I lived in fla, speeding tickets started out at $160 [?] and then added $10 for each mile over the limit I figured that at my wages - I best not go fast enough to get a ticket
 
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Old 04-05-10, 08:22 AM
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Originally Posted by ecman51` View Post
Seriously(again)? Gee, what if a person then had posi-trac and one tire had a bigger circumference than another like-marked tire? You sure about this? Why do they bother labeling tire sizes then?
I probably wasnt clear in my statement. What I ment was different brands with the same size markings will vary in size.
 
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Old 04-05-10, 08:47 AM
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ecman....
Something was wrong with your calculations....

60mph is a mile every 60 seconds. 30mph is a mile in 120 sec. No way 56-57mph is 103.5 sec. 56-57 kph yes...mph no...
Speed Distance Time Calculators
 
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Old 04-05-10, 03:34 PM
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Originally Posted by retired wrench View Post
I probably wasnt clear in my statement. What I ment was different brands with the same size markings will vary in size.
No, I followed that. So what good do the numbers do then? I thought they were industry standards?
 
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Old 04-05-10, 04:14 PM
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A few things to keep in mind....

Absolutely correct that no two tires are the same diameter. Ive seen two brands in the same size that are DRASTICALLY different due to construction and tread design.Even a variation of 3 to 4 psi can make a diameter difference in a Tire.

If your Car was a CANADA build...it is possible ..from the late 70's , all the way to 98 or 99, Your Odometer may be METRIC.....

Now the tricky part....Starting in 1990, Chrysler introduced the TCM, (Trans control Module) on virtually all of its "High End" cars. By 1991 or 92, it would have been standard equipment on All 3speed Automatics. Aside from the obvious function, The large Upside to this Technology was to allow the Technician to "Program " what is called PINION FACTOR" and Tire size, into the TCM, allowing almost Infinite flexibility. A dodge Spirit and A Caravan , would use the Same TCM, with the Unique details of each programmed into the TCM. Downside was that The Speedo No longer came from the Drive train. A speed signal was generated inside the transmission, With No help from the wheels themselves. This signal was "Assumed" to be accurate based on the criteria that was Programmed into TCM. ANY variation from design, or Corrupt info, would destroy its accuracy.( Funny Story...Ive seen a Caravan rack up 335 miles in 10 minutes-------BAd alternator Diode Allowing AC voltage to pass to the Cluster.......)
 
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Old 04-05-10, 04:19 PM
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Originally Posted by Gunguy45 View Post
ecman....
Something was wrong with your calculations....

60mph is a mile every 60 seconds. 30mph is a mile in 120 sec. No way 56-57mph is 103.5 sec. 56-57 kph yes...mph no...
Speed Distance Time Calculators
I can't do the calc from your site because as soon as I click my mouse to enter 1 mile, the computer feezes up! Aaaaargh!!! Have you tried to plug in my numbers?

This is how I did the math by pen:

60/2minutes = 30mph.

60/1.058333 minutes = 56.7mph

60/1minute = 60 mph

So where did I er? I can't see it?

Here is a more detailed answer:

1 mile in 2 minutes = same as 60 miles in 120 minutes = 30 miles in 60 minutes(an hour)

1 mile in 1.05833 minutes(or 63.5 seconds) = same as 60 miles in 63.5 minutes = 56.7 miles in 60 minutes (an hour)

1 mile in 1 minute = same as 60 miles in 60 minutes(an hour)
 
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Old 04-05-10, 04:29 PM
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Originally Posted by Unclediezel View Post
A few things to keep in mind....

Absolutely correct that no two tires are the same diameter. Ive seen two brands in the same size that are DRASTICALLY different due to construction and tread design.
If a person had posi-trac I guess you would not want to install just one new tire on a drive wheel then. And even if you did not have that feature, the car could sit slanted some, eh?

While we are on that subject, ever hear of anyone putting slightly bigger diameter tires on the right side of the car to try to offset the crown in the road?
 
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Old 04-05-10, 04:33 PM
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Well...yer post said 103.5 seconds which is not 1.058333 min...if you meant 63.5 seconds (1.058333 min)..then you are correct. It was about 56.7 mph.

Also..the "Your Speed is" trailers out here are pretty accurate from my experience. Maybe different back east...but they are really close here. I even play with them if there is no traffic and they match pretty well to my speedo.
 
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Old 04-05-10, 04:43 PM
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For the AWD guys out there....Yes, the owners manual DOES Recommend changing tires in "SETS OF 4"....

ECMAN.....Yes, you should replace the pair on a Locking Diff.
I dont know about road crown, but Ive seen Circle dirt track cars do that.....It draggs the car in the direction of the smaller wheel.....
 
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Old 04-05-10, 05:04 PM
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Originally Posted by Gunguy45 View Post
Well...yer post said 103.5 seconds which is not 1.058333 min...if you meant 63.5 seconds (1.058333 min)..then you are correct. It was about 56.7 mph.

Also..the "Your Speed is" trailers out here are pretty accurate from my experience. Maybe different back east...but they are really close here. I even play with them if there is no traffic and they match pretty well to my speedo.
Achhhh. Ya. So sorry to put you through that work. On my stop watch it came out 1:03.5 minutes. Sorry!

About those trailers. I was thinking that they'd be pretty accurate also. Otherwise, why would law enforcement have them? People could use them to argue their way out of a fine by saying that they used that trailer to determine hiow fast they are really going.
 
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Old 04-05-10, 05:08 PM
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Originally Posted by Unclediezel View Post
ECMAN.....Yes, you should replace the pair on a Locking Diff.
I dont know about road crown, but Ive seen Circle dirt track cars do that.....It draggs the car in the direction of the smaller wheel.....
Like 'The World of Outlaws' racers.
 
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Old 04-05-10, 05:19 PM
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Exactly.......................
 
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Old 04-06-10, 07:39 AM
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spedo

Can't say currently but back in the day there were no industry standards on tire size.
 
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Old 04-06-10, 10:54 AM
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AWD Vehicles

Here's something my daughter told me:

"Tires on AWD vehicles MUST be exact matching make and model and they have to be very close in depth. Most manufacturers say 2/32 but some go as far as 3/32. You cannot mix and match tire brands or damage to the drive train may occur."


I had never heard of such a thing. When I was a youngster we'd put on anything that would hold air, who cares what size it was.
 
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Old 04-06-10, 11:28 AM
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Originally Posted by Baldwin View Post
I had never heard of such a thing. When I was a youngster we'd put on anything that would hold air, who cares what size it was.
That makes two of us. And for a while, I was going to the used tire store to hopefully get ONE tire, mounted and all, for like $10. They actually had some tires with good tread on them yet, which I had a hard time figuring out(from the fact I was poor and cheap). But they claimed there are people out there that are real fussy on keeping full-tread/new tires on their cars at all times.
 
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Old 04-06-10, 01:28 PM
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4x4 and AWD are different than the cars of old. When giving power to all 4 tires you want them to rotate at the same speed.

If the front tires are rotating faster/slower than the back and they are all hooked to the same transmision/transfer case - you can see where the added wear and stress would come from. Even on the old rear wheel drive it wasn't good for the rear end to have 2 different size tires on the same axle.
 
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Old 04-06-10, 03:25 PM
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Originally Posted by Baldwin View Post


I had never heard of such a thing. When I was a youngster we'd put on anything that would hold air, who cares what size it was.
2 billion and a half percent correct.-

At Chrysler, for argument, we have the Pacificas in FWD and AWD configurations. The AWD uses a 19 inch Michellin, which is the only MFR that produces that specific tire. Needless to say, Supply and demand forces the price upwards of 235$ per tire. And they must be changed as 4 pcs!!!!!!

I can PROMISE YOU...if you only change 1 or 2 at a time, Inside 100 miles, your Transfer case (We call it a PTU--Power transfer unit) WILL be Blown to bite sized pieces no larger than a Dime.
 
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Old 04-07-10, 05:37 PM
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I can PROMISE YOU...if you only change 1 or 2 at a time, Inside 100 miles, your Transfer case (We call it a PTU--Power transfer unit) WILL be Blown to bite sized pieces no larger than a Dime.

Wow! Without getting too technical could you explain why this happens? Unless I win the powerball I'm sure I'll never own one, just curious.

Thanks
 
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Old 04-07-10, 06:44 PM
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If you have ever seen a Tractor Trailer make a slow sharp turn, you will notice that the rear most wheels that are on the "Inside" of the turn radius can appear to turn "Backwards".

On a basic rear wheel drive car , say 70's or 80's, there is a differential, To make up the "Differential" in rotational speeds between inner and outer wheels of a turn radius.

Just like any other mechanical device, Constant use shortens its life span. If you were to drive in a circle constantly, your differential would be stressed and wear out. Using 2 different size tires, puts the same type of stress on a differential as constant turning, the "Larger" wheel will turn slower than the other.

Now the trick.....couple the wear on a differential from one side to the other, with an equal force from front to back.....???? You basically have two differntials , spinning at two different speeds, and , at the same time ,trying to compensate for left to right variations.

Maybe 100 miles was a bit overexaggerated....but you get the Idea...
 
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Old 04-07-10, 07:07 PM
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Ecman, I use my Gps as my spedo most of the time, because of tire size differences my spedo is about 3.5 mph at highway speeds, that means the odometer is also off but cant change that, I have checked it numerous times with roadside radars and the gps reads spot on.
and so what if people want to speed, I just drive the speedlimit and don't block the left lane, (most of the time) although if you tailgate me I will probably brake check you-lol
murphy was an optimist
 
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Old 04-08-10, 11:39 AM
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It is amazing how oblivious people are when they are driving under the speed limit in the passing lane until you pass them on the right. Then they high beam you even if you leave 1/10 of a mile before you hop back into the left lane. Also wonder why even when highway traffic is very light why people go into the left lane going 60 mph (65 limit) a mile before a leftg exit.
 
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