I went from factory 31's to Dunlop 35's (34" measured). I sit in traffic a lot so I have had ample opportunity to measure actual vs. odometer mileage. I found that my odometer is off by .5mi every 10 miles. Now if I take that measurement and figure it against a 60mph drive, i would actually travel 57 miles, making my speedo off by -3 mph, which I have also found to be the case. The question remains: why isn't my odo/speedo as far off as some I have seen? The only thing I can think of is that I have ABS and the odo/speedo may be taking a measurement at the tone ring(on the spindle/bms) and making a calculation based upon that input. Anyone else running ABS and larger tires find this to be the case?dougtrd said:3. putting bigger tires on will cause your odometer to read less than actual miles causing your milage calculations to be off. get a GPS(buy or borrow) drive 100 miles according to the odo, then check the GPS for ACTUAL miles driven. then you can figure a % variance and calculate mileage better and be aware of the odometer variance to know the actual miles on your truck etc..etc..etc.. You might be surprised to find out how wrong your ##'s are esp with 33's or bigger!!!