Is the speedometer based off front or rear tire size?

[Andy13186]

Im getting some larger rear tires and wonder if the speedometer is based off front or rear tire size?

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[Nagare]

Rear

 

[HoosierDaddy]

FWIW, I believe the tire size used to calculate speedometer values can be changed with Forscan.

 

[Rjames18]

Do a burnout and you'll find out really quick lol

 

[ahl395]

Rear

You can change the tire size to fix your speedometer via Forscan or a Whipple flare tool.

 

[MKMotorsport]

SCT X4 handheld can adjust tire revs per mile as can SCT PRP.

 

[Andy13186]

Its possible to change with the Nguage too. Works on the PBD nguage

For some reason my tuner thinks the computer will auto adjust for tire size? Anyone heard of that before?

 

[RIBS]

Wow, NGauge uses Tire height? That can vary depending on weight load and tire pressure. They should use tire Circumference, which should stay the same, and only change as rubber is worn off.....

 

[HoosierDaddy]

Wow, NGauge uses Tire height? That can vary depending on weight load and tire pressure. They should use tire Circumference, which should stay the same, and only change as rubber is worn off.....

I suspect they do that because tire height is a manufacturer number easily found with a web search. Measuring actual circumference would be harder and more prone to human error.

 

[samd1351]

Wow, NGauge uses Tire height? That can vary depending on weight load and tire pressure. They should use tire Circumference, which should stay the same, and only change as rubber is worn off.....

The circumference of circle (the tire) is calculated as 2 x pi x r (radius) or pi x d (diameter). If you change the height (diameter) the circumference will change as well. As HoosierDaddy said, tire height (diameter) is a published number for almost all tires and easily found.

 

[M151A2]

Your tire size does have an effect on the Speedo but it' is not going to be more than about 2-3 MPH at 70MPH.
Unless you're running monster mudders or something crazy like that, I wouldn't worry much about it.
Most LEOs will not bother you for a 2-3 MPH over the limit speed and it's entirely possible that the error factor in your speedo is somewhere in that MPH range as well.

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