I haven't changed my speedo gear out yet and thought my mileage had declined drastically with the new 31X10.50R15's. After considering it a bit, I gathered RPM data for the old Michelin LTX LT 235/75R15's and the new Bridgestone tires from thier respective web-sites. I divided the new RPMs by the old. => 681/727 = ~0.937 I then divided the odometer reading by this number to get the actual milage. For example, a odometer reading of 275 miles is really about 294 miles. This will change my MPG calculation by 1 MPG. I am now getting about 16 - 17 MPG (adjusted). I used to get 17 - 19 on the old tires. Of course my driving style changes that drastically if I start getting lead-footed.

I should really get that 24-tooth speedo gear per DougB(?).

You are running 35's though. I would assume 10 MPG is pretty good?

I would roughly calculate the RPMs on 35's at 585. For the example above, divide 250 by 0.805 (585/727). The substitution would be about 18 MPG, all other values unchanged.

I dunno...