Can a speedometer and tach regsiter differently on different days?

Same Dodge car.

I have been suspicious of this for quite some time now. Years ago, when I had a different vehicle, I started counting the seconds between electric poles along the highway, to gauge my speed in case my speedometer went out, as would occasionally happen in the truck I used to have (which did not have a tach, as a backup).

I would count about 4 seconds at 60 mph.

Now with this Dodge, one day I am counting 4.5 seconds, and now this morning, I felt like I was going extra fast, by the rpm sound of the car and the tire sound on the road and the speed at which the lane stripes were flying by. Yet my speedometer and tach read same as normal.

So I counted between electric poles and got about 3 seconds!

I am a pretty good counter. I know I am. I do not believe I am varing my count speed that drastically. Therefore, do you know if it is impossible the way these speedometers (and maybe tachs) are set up today (and with mine, both may be affected), that something can change to make it read different from one day to the next? Something about simply starting the car, and the computer and all that?

If it can, I am going to have to rely on driving instinct and pole-counting, more than the gauge, I guess. I'm not really fond of being pulled over.

 

you are guessing. take a timer and TIME those poles. time them maybe 20 times in a row to get median. you can't just count seconds in your mind and think it's a good count. it's flawed. and this is coming from a guy who's been weightlifting since 27, 53 now, and i always count seconds between sets. and even after all that counting, if i glance at the gym clock, i'm always off on the second handle. use SCIENTIFIC TOOLS, before jumping to conclusions.

 

I have checked my counting ability, and found I am pretty close, always. Being 1/1/2 seconds off in a 4-5 second range is a huge - HUGE - percent disparity.

I do know for one thing that my tach has changed on me. It used to read 2500-2700 RPM's while my speedometer read 62. Now it reads about 2300 (with the same size, mfg. suggested tires on the car). That is why I need to better understand how speed and rpm is 'read' on these computer era cars, as compared to say the 60's cars. Then I'd probably know right away if what I have said is even possible or not.

 

Bite the bullet and get a GPS. That will give you the truest speed and distance.

 

Interesting idea.They do MPH? Didn't now that. These things probably cost more than my car is worth. Although I guess you can take it out of one car and put in the next.

 

If it is that important, get them both calibrated. Or, you could slow down and not worry!!

 

But my real question is not how the calibration is. I want to know if it is electronically even possible that one day it can say I'm going 60 when I really am going 60, and another day I can actually go 64 on my speedometer, but actually be doing 60. (Remember - I already know my tach is screwy, as that use to read about 2700 and now reads 2300.

This is REALLY important to me, as where I drive on the highway, miles daily, there are lots of no passing zones, and people can get po'd with you if you do not go 1/2 mile an hour under what cops will pull you over for. Going 58 is not good enough for these people. We have road rage people who who tailgate you, take cuts, run complete red lights, etc. I have to know the answer to this. It will not help if I just go past a roadside radar gun, unless, I suppose, I try to locate these things several times and see.

From the theory aspect, I'd simply like to know if it is possible for it to change from one day to another.