Does size of tires affect speedometer reading?

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Old 04-07-06, 07:46 AM
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Does size of tires affect speedometer reading?

My son recently bought a 1994 Toyota pickup. The tires are not the original tires instead they are Brute Mud Grip tires and are larger than the original street tires. Being that they are larger tires - does this have any affect on the speedometer readings? Thank you.
 
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Old 04-07-06, 07:51 AM
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Yup, sure does
 
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Old 04-07-06, 07:53 AM
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The speedo runs off a little gear in the transmission. This can be changed for different sizes of wheels, on some vehicles anyway.
 
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Old 04-07-06, 08:04 AM
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If he takes a reading on the odometer with the interstate mile markers he can get a rough guess on how much the speedo is off. If it only registers .9 of mile between markers then it is off approx 10% = 50mph is really 55.

Most areas have atleast 1 shop [often new car dealer] that will calibrate speedometers.
 
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Old 04-07-06, 10:41 AM
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Interesting technical question. Always knew it affected the reading, but given that nearly all speedos are now electronic, are they calibrated by reprogramming the computer?
 
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Old 04-07-06, 01:04 PM
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Originally Posted by the_tow_guy
....but given that nearly all speedos are now electronic, are they calibrated by reprogramming the computer?
You mean can you re-calibrate?
That is an interesting question
I'm used to the older cars and switching the speedo gear, which wreckwriter is right, some ya can, some ya can't

I believe on this vintage Toy it's a gear, but for some reason they recommend replacing the "assembly"
Most people I know just +/- the speedo reading in their heads
Fact is, tires wear out and you might want to go stock next time
 
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Old 04-07-06, 03:13 PM
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Computer controlled speedometers can be calibrated, but they do not do it on their own, has to be taken to a shop for this.
 

Last edited by stickshift; 04-07-06 at 05:06 PM.
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Old 04-07-06, 04:28 PM
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it being a 94 It probably cannot be programed but a gear change on the vehicle speed sensor is often used to correct a speedometer, alot of later models im sure can be programed if they take it to a dealer.
larger tires will cause the speedometer to read lower than the actual speed per example speedometer may read 60 but the actual speed may be 65.
 
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Old 04-07-06, 04:54 PM
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Sometimes you can attach a gear directly to the speedo cable [@ trans] to modify [fix] the speed discrepincy.
 
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Old 04-07-06, 05:01 PM
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Originally Posted by mfrancle
My son recently bought a 1994 Toyota pickup. The tires are not the original tires instead they are Brute Mud Grip tires and are larger than the original street tires. Being that they are larger tires - does this have any affect on the speedometer readings? Thank you.
Yes it does the frist ticket I EVER GOT WAS BECAUSE i PUT BIGGER TIRES ON A fORD fALCON, Thought it looked cool but costly
 
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Old 04-07-06, 08:33 PM
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revolutions

yeah it does although minimally unless tires are way different form the stock ones in size, my local ted wiens store does calibrate speedo for different tire sizes.
 
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Old 04-29-06, 12:47 PM
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