Going off of what master John said, other than a GPS the most accurate test for the odo is the mile markers on the interstate (go five miles) and the most accurate test for the speedo is the temporary roadside lighted digital signs that are set out on occaision. Get the right tooth pinion to get the odo correct THEN if the speedo ain't correct at that point in time right along with it then the speedo head needs to come out & go to a speedometer specialty shop to be recalibrated. EDIT If they ain't equal & I dont want to send out the speedo head then if I can get the right toother in there to get the speedo correct then that is what I prefer (helps with avoiding tickets)