I put a piece of stock across the top of a rear tire and measure my actual dia to the pavement and w the ratio known, picked the pinion from the MP archives chart and it made my odo and speedo dead on. Checked the odo on the interstate mile markers and the speedo at a digital road side sign. The odo can be spot on w the correct pinion and the speedo still off if the magnetic speedo gismo is not calibrated right & luckily mine was (& most are)