Speedometer correct at 40 but off at 65?

[Asylumwarp]

As the subject reads. Working on a 2000 Silverado 5.3 with stock gearing. Tire height been changed to a smaller tire. So ive gotten the mph correct at 40 mph according to gps, but when we speed up to 70, the gps is reading 63. Somehow its gaining mph. Anyone seen this happen? The gps I'm using seems to be correct in other vehicles tested in different cars, different speeds. Its in the tune.. just where?

Thanks

[2xLS1]

Are you comparing GPS MPH to cluster MPH or scanner MPH?

[mecanicman]

Your changing a wrong value and just about doubling another. Post your gear and tire size.

[Asylumwarp]

I'm trying to fix cluster mph. I don't care about scanner mph, as it matches the cluster. It's 27" tall tire with a 3.73 stock rearend. I had changed other values but it didn't seem to change the cluster. What are you guys seeing?

[mecanicman]

Compare to speedo-general tab, this should have you pretty close.

[Asylumwarp]

Hmmm.... I see what you did. And checking a few stock files, its close to the same settings. I'm not the first person to tune this vehicle, and once I was done fixing the tune the customer asked me if I could correct his speedometer.. I just changed the top number accordingly to get the speedo correct. Why someone changed the 4000 is beyond me. I don't do much speed calibrations. Kind of new area to me. Ill go try these settings and report back. Thanks

[Asylumwarp]

Ok, so the speedometer is now consistant at speeds. But its slow about 6-7mph.

[mecanicman]

So the tires are not truely 27" Apply the error from the speed difference to vss ppm and trans revolutions per mile and you will be good.