MPH Accuracy

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Oct 25, 2023 | 05:09 PM

C8gofast

Thread Starter

Racer

Joined: Mar 2005

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MPH Accuracy

The OEM 4S tire has a perimeter of 85 inches. The Pirelli tire I am using has a perimeter of 87.5 inches on the rear.

What sensor determines the mph displayed? Is it from the gearing/ transmission, wheel sensor?

If the displayed speed is 157mph at Road Atlanta am I going 157 or 87.5/85= 1.03 x 157 = 162mph ?

Oct 25, 2023 | 06:20 PM

IOSProKC

Instructor

Joined: Aug 2023

Posts: 179

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From: Overland Park, KS

A bigger tire has a larger circumference, thus traveling a further distance for each revolution. A speedometer reads the amount of revolutions your tires are making, not how far you are traveling. Therefore you are actually traveling faster than your speedometer says when you have larger tires than stock, and slower than you're speedometer says if you have smaller tires. So, your calculations are reasonably correct.

Are the tires different 305/30R20 vs ???
You can get the exact info here: https://tiresize.com/comparison/

Oct 26, 2023 | 10:00 AM

Andybump

Race Director

Joined: Mar 2007

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Quote:

Originally Posted by C8gofast

I think I understand you question. You are asking if the speedometer reading is determined by the speed of the rear tires. I found this in the Service Manual that suggest that the speed is calculated from the engine crankshaft speed, therefore, yes, it knows the selected gear ration in the transaxle and makes an assumptions about the diameter of the rear tires - so larger rear tires will cause it to read low. I did, however, note that there also individual wheel speed sensors that are used in the ABS and maybe other automated handling aids. And, I ran across a DTC that can be set if the vehicle speed sensor and a wheel speed sensor do not match. One of the conditions that causes it is incorrect tire/wheel combination. I have never seen anyone report an issue like that with different sized tires - so there must be some level of tolerance on setting of that code.

Last edited by Andybump; Oct 26, 2023 at 10:54 AM.

Oct 30, 2023 | 11:11 PM

Red Mist Rulz

Race Director

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If it's using crankshaft revolutions to determine vehicle speed, it has to calculate it differently for each gear. That somehow seems a complicated way to do it when they could calculate it directly from either the transmission output or the ABS wheel speed sensors.

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