Speedometer correction + gauge accuracy

[Crispy]

Hi all, just recently got back into the OBS game with a 2000 Tahoe Z71. Its has a great prior service record and is a well taken care of 2 owner vehicle. I bought it with 265/70/16 tires and the door tag calls for 265/75/16. Seeing as the tires are like new I bought a superchips 2815 tuner to correct the speedometer since $125 for a used tuner is far cheaper than another set of tires.

My issue is that the speedo is still off. I've checked my speed the following ways:
Ulysse Speedometer Pro app for android
Torque Pro app for android
Running side by side to a Honda Civic with those big digital speed displays

The interesting bit about using the torque app is that my speedometer reading, GPS speed, and PCM reported speeds are all different. Does anyone have any suggestions on the next step in diagnosing and repairing this problem aside from buying a new set of tires? I know the GMT400 isn't known for great instrumentation but I'd like to think it can be better.

Side notes:
speedo rests at 0 while stopped
no check engine light
no ABS or Brake lights on the cluster
RPO code calls for 3.73 gears (have not confirmed, but no junkyard marks on diff to indicate replacement)
Speedo is off by a percentage not a set MPH. When the speedo shows 30 I'm doing 28-29 when it shows 80 i'm doing 68-69

 

[Spastik Monkey]

IIRC, a factory speedo can by off by up to 10% and still be "correct". It's the nature of the beast. My 2015 traverse is off 3mph at 75mph.

 

[GMRedline]

It's pretty easy to physically adjust the position of the speedometer needle by removing the gauge cluster and pulling the front cover.

With the gauge and cover removed, set the gauge on a flat surface and move the needle to 60mph (or whatever). Then use a pry tool to carefully remove the needle. Then reinstall the needle at the proper adjusted speed. So if the gauge says your going 60 your actually going 64, remove the needle at 60 and reinstall at 64. Kind of redneck but it should work.

OR

You could find someone with HP Tuners or EF Live and they can adjust the tune to your proper tire size.

 

[Crispy]

I understand a factory discrepancy but damn 10% is a lot lol.

I dont think removing and replacing the gauge will work since my speedo reads faster than my actual speed. Adjusting the gauge position will cause the needle to want to go past the rest point at 0 mph.

I may have a line on someone buying my tires so I can get correct ones, hopefully that will fix it. When I go HPT or EFI live it will be to adjust way more than my speedo.

Thanks for your input guys.