Speedometer is 10 miles off

[Mus19tang]

I have a 2000 Ford Mustang GT and I changed the gears in the rear end and now my speedometer is 10mph off. Any suggestions how I can fix that? I put 3.73 gears in it. That’s what I changed it to.

 

[gkomo]

I'm not sure if your year Mustang can just have the speedo gear in the transmission replaced or not. If so, they make speedo gears for 3.73's (and all ratios) to correct the speedo.

If a 2000 Mustang doesn't use those speedo gears and its all done electronically, then i think you need some sort of electronic calibration tool (like an OBDII tool) to correct it via the computer.