Speedometer error

I have a 1986 mustang with a 5 speed manual tranny. I have changed the speedometer and the speedo cable, but it still reads wrong. A couple of years ago, I replaced the original 5 speed tranny with a new tranny from ford racing, so it was a direct bolt in, but some of the gear ratios were different than the original tranny. Could this be the cause of the speedo error since the speedo cable is attached directly to the tranny? Everything else is original including the differential.

It has nothing to do with the gear ratios. The final drive (output shaft) is what counts. Any other info will have to come from somebody else.

This maybe goes without saying, but you didn’t mention anything about the tires! Are they still the same size? That’s the most common cause of inaccurate speedometers. Also, does it read consistently wrong, i.e. always 10% off or something, or does it give erratic readings?

If your town has a speedometer shop, they should have no problem recalibrating it.

The speedometer drive gear needs to be matched the final drive ratio (tires and differential). This gear is what the speedometer cable plugs into at the transmission end.

Just find a speedometer shop in your area. Carefully measure both the speed and odometer readings and give them to the shop. They need to do the adjustments by replacing some parts in the speedometer.

If it’s reading 2 to 4% high, that is a common occurence…a designed safety feature?..and usualyy not fixable.

You say your car still has a cable operated speedometer. That was just about the time they changed over to electric speedos with vehicle speed sensors instead. There is a plastic gear in the tranny that the square end of your speedo cable plugs into. You didn’t say how far off the speedo was, or which way it was off. You need to pull that little gear out and count the number of cogs it has. IIRC it has a c-clip that holds it in place in the tranny. If your speedo is reading too slow, you need a gear with fewer cogs. If you’re reading high, you need a gear with more cogs so it will turn more slowly. Once you have the gear out, the Ford parts counter person can sell you the one you need by reading the number on the gear.