MPH Accuracy
Thread Starter
Racer
Joined: Mar 2005
Posts: 410
Likes: 288
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 ?
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 ?
Instructor
Joined: Aug 2023
Posts: 180
Likes: 153
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/
Are the tires different 305/30R20 vs ???
You can get the exact info here: https://tiresize.com/comparison/
Race Director
Joined: Mar 2007
Posts: 11,375
Likes: 8,532
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 ?
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 ?
Last edited by Andybump; Oct 26, 2023 at 10:54 AM.
Race Director
Joined: Sep 2016
Posts: 13,180
Likes: 10,523
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.
