02 Trailblazer, larger tires - cannot correct speedo/cluster but computer is correct

[Excell]

Measured rollout and completed the math to adjust for my larger tires:

Tire Size in inches: 31.03105096
Gear Ratio: 3.42
VSS Teeth: 40
Tire Circumference: 97.4375
Tire Revs/Mile: 650.2629891
PPM: 88955.97691
Output Shaft Revs/mile: 2223.899423

With the stock file, both the computer and speedometer/gauge cluster read ~7.75% off which is expected.

When I put these settings into the VCM with a calibration only write, the computer reads correct to GPS but the speedometer/gauge cluster are still off by ~7.75%. What am I missing?

Thanks for your help!

[Excell]

Having trouble with the uploading feature of the forum, so I hosted the stock and modified tables:

ChrisGreen_12587430_stock.hpt
ChrisGreen_12587430_32tires.hpt

Thank you.

[Excell]

I just looked at a file and there is a spread on the values. does your stock tune have a spread?

That is the stock file and it does have a spread. The stock file is "ChrisGreen_12587430_stock.hpt" and the modified file is "ChrisGreen_12587430_32tires.hpt".

I actually reverse-calculated the spread so I could program a file with two different values like stock, but I've read several threads on here where HPT Chris says they should be the same. For kicks I did write a file with the reverse-calculated spread to the VCM and there was no change - computer still correct, speedo/gauge still off.

[Excell]

Thanks for calculating those. With those numbers my computer is off by ~1% and the speedometer/gauge is still off by ~7.75%. I expected this would happen as the difference in your numbers are only slightly different than mine, and the speedometer remains unaffected.

Is it necessary to write the entire file for a change like this? I've only been writing the calibration.

[Excell]

no.. could be an error with the dash calibration.. adjust your settings by the 7.75% and it should fix it

When I do, the computer is off by 7.75%.

[Excell]

one setting is for the dash.. one is for the puter... open the editor/speedo hit F1

It's PPM. When I do, the computer becomes off and there is no change in the speedo.

[Programmer]

The required parameters are not in HPT. You can't adjust your speedo, or ABS pulse.

[Excell]

The required parameters are not in HPT. You can't adjust your speedo, or ABS pulse.

So weak. I had a feeling this might be the case. How are the various tuners (like yourself ) correcting this then? EFI Live?

[Programmer]

Our own software...no, we don't use EFILive. I don't know if they have it right, either.

[Russ K]

http://www.hptuners.com/forum/showthread.php?t=5058

Russ Kemp