Tire/speedo Question

[JetBrett]

Does anyone know whether the speedometer reads from the front tires, the rear tires or both. I'm going to a 18' rim (285/35/18 or 305/35/18) at the rears, but am going to keep the 17's (245/45/17) up front. The tire calculators I've looked at show that the speedometer will be very slightly off, but I don't know if the calculator's assume all four rims/tires are the same size.

Is anyone running a similar set up and if so, how accurate is your speedometer?

 

[Juiced87Notch]

it reads off of the rears only.

 

[JetBrett]

it reads off of the rears only.

Got it. Thanks

 

[Juiced87Notch]

Got it. Thanks

n/p, just a guess, but I'd say your spedo should be about 5mph off, if that.

 

[281pony]

try this for calculating the difference if you didn't.

http://www.miata.net/garage/tirecalc.html