My truck came with 265's. I am running 315's now. I am getting 500 miles per tank on the OD. What do I multiply my miliage by?

The 315's are roughly 10% bigger than the 265's so what ever your trip reads add 10% to that and divide by gallons of fuel.

need 2nd #

265x75x16....to....315x75x16 would be 9.3%

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

Thanks guys.

Why not just get your CPU reflashed at the dealer? Then you would have the correct Odometer and Speedometer readings.