car in a parking lot with wheels straight ahead. put a paint mark on a rear tire at 8 o'clock. roll car forward till you have two paint marks. tape measure that straight distance which is "rollout". divide Pi into that number to get the actual "diameter" that the speedo will see. there's a chart that shows the correct pinion teeth for tire diameters down to the tenths of an inch. I have it but it wont post as it keeps telling me it needs to be a jpg extension or some nonsense . jack up the car high on the dr side to minimize the fluid loss. be sure to clock the adapter correctly for the number of teeth your pinion going in will have. find that chart (or show me how to post it). You need correct diameter & rear gear ratio