My 91 F250 has 35" tires. My speedomiter has not been adjusted to compensate. Using the road mile markers i found my odomiter reads 9/10th of a mile for every mile marker. So I can asume my odomiter is off by 10%? this should be the same for the speedomiter? So if my speeedomiter reads 60 mph I am actually traveling 66 mph? Has anyone done a speedomiter adjustment for larger tires? What does it involve? Thanks for any info.