I'm preparing to do the speedo correction for my Dad, he's bumping up to 3.73 rear end ratio in a couple weeks. He has a 94 z28 with the A4 (My car is a C5 vette, hence my membership here, but I figure you Lt1 C4 guys must have run into this question at some point).

First off, I'm determined to do this using TunerPro rather than TunerCat or any paid solution that generates the numbers for me. My goal is to actually learn how the things are working under the hood, and not to save money, so I'm not interested in advice to use TunerCat and let it spit out numbers for me.

I've already determined all the numbers I should put into each constant under "Instrumentation Calibration" and I'm very confident my numbers are all good, except 1. I'm using the tooltips when I mouse over the parameters, which I believe are provided from the XDF from Steveo at EEHACK.

So here's my question:

For "Speedometer Scalar - Divisor" , the tooltip says to enter:

4000 / ( PPM / "speedometer scalar - base" )

However, we know that

"speedometer scalar - base" = ppm / 4000

So that means that "Speedometer Scalar - Divisor" can be re-written as:

4000 / ( ppm / ( ppm / 4000 ) )

which simplifies down to 4000 / 4000 because the PPM cancel out. And...

4000 / 4000 = 1

This tooltip gave me a function that is algebraically guaranteed to always be exactly 1, independent of the PPM. Not only does this seem too useless for any GM engineer to put as a parameter in the computer, but I can see that the default value already present is 1.0097. So, there must be some other member of the function used to determine this value, which isn't present in the tooltip.

How do you determine ""Speedometer Scalar - Divisor" ???