If I remember correctly, the op car is a 1973? No matter what transmission he uses it will have to be set up to work with the instrument cluster that he has, such as the speedometer. I'm sure that you know that if you change the size of your rear tires that it affects the way the speedometer works, same thing with a gear ratio change. I never said anything about the engine what so ever. If you still think that I'm wrong, drive your vehicle as is for a couple of miles, then put on either a taller or shorter height rear tire and drive it again the same amount of distance and see how far off the speedometer is off.