1998 mustang gt 18" staggered wheels/3.73 gear combo speedometer calibration help!

Hi, Apologize my battery is about dead. Call American Muscle & Ask BAMA about your request, they’ll take care of your final output ratio, you can combine both app’s mentioned, the Math is quite easy, easiest to get the Digital take from an SCTX4 Tuner, tunes for life- free. The meter will easily be able to compensate for the difference.
I’ll pop back on with the equation if you require it, just ask. Should be powered up by morning, already 5AM. A few hours, happy to help. Call AM first, then we can work it out further if need be.
Good luck!
-John

There's a fundamental difference between the 1996-1998 and the 1999+ that IMO is important that will help you to understand WHY there's confusion.

The 1996-1998 Mustang uses a VSS speed sensor. The OUTPUT of the VSS sensors goes to both the PCM AND the cluster.

Whereas the 1999+ uses an OSS sensor. The OUTPUT of the OSS sensor goes to ONLY the PCM. The PCM decodes the sensor output and "learns" the car's speed. The PCM then "tells" the cluster what speed to display.

Regarding why there is an apparent contradiction in what the forums say. Think about what would happen if just a tuner were used on a 1996-1998. This would create a situation where the PCM had the correct speed but the cluster would not.

But for the 1999+, if the PCM is altered using a turner then the PCM has the correct speed. But since the PCM now "knows" the correct speed it "tells" the cluster the correct speed.

Soooooooooo. This means for the 1996-1998, a turner alone can't do the whole job. Either a different VSS gear or an external converter box is needed just downstream of the VSS sensor. This allows the PCM and the cluster to benefit from the altered signal.

For the 1996-1998 model year a single solution isn't really possible. Just consider the rear end correction and tuning for performance different problems and work from there.