speedometer/odometer reading low
 

I have a 1966 Mustang, that has a speedometer that is reading about 25% low. For example when a drive a known distance, in this case 80 miles, the odometer indicates 60 miles, and the speedometer is also indicating low. I have replaced the plastic gear with the one the factory calls for, with the same results. It has 16 teeth. It would appear that I need one with LESS teeth, but I have not been able to find any with less teeth. I have found units with up to 20 teeth, but this would make the situation worse. Engine, 289; transmission C-4 (not sure of year), diff 2.80/1 gears. I have factory styled steel 14 inch wheels on the car, with radials very close to the original tire size.

Any ideas would be greatly appreciated.

My stang reads too high, want to trade speedometers?-lol

If your about the stock size on tires, and u do have the stock 2:80 gears, then I'm lost. I guess u need a new speedometer. Not sure. I'm putting on bigger tires to fix my problem.

I have just found out there were three variations on the C-4 transmission, where the shaft gear has 8-9-10 teeth, or 7-8-9. In an case, I need to check the transmission, maybe I do not have the 'original' transmission. I did not pay attention to this during the rebuild, as it was functional.......

Tires are very close to the factory original in terms of the outside circumfrence.

Back to the drawing board.