Speedometer correction + gauge accuracy

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

either a funky speed sensor or the tire setting or axle ratio settings in the pcm are skewed.

I have a similar isssue with mine. Torque, Ulysse, and Autel Maxicheck Pro all read correct speed, but my speedometer is reading faster than acutal. Like yours, mine is within 1 mile between 30 and 40. When I'm going 50, the needle says 57. When I'm going 65, the needle is past 80. I sent the cluster off for repair. They adjusted the speedometer but it is still incorrect. I wonder if something is haywire in the stepper motor.

then it does not sound like they repaired it. have them rework it.

FYI, I got the correct tire size for my Tahoe (265/75/16) and the speedo is now off by 2-4 mph. Much better and I guess acceptable for now in my book.

As for your issue eswarden, these stepper motors are know be be finicky but can you confirm that you have the OEM recommended tire size or that the computer hasn't been tuned to change that?

Tire size is larger than stock and the ECM has been configured by Blackbear for the tire change. However, the speedometer reading was off long before any of that took place. It's weird because if I'm doing 20 it reads 15, but when I'm doing 60 it reads 70.

Crispy said:

FYI, I got the correct tire size for my Tahoe (265/75/16) and the speedo is now off by 2-4 mph. Much better and I guess acceptable for now in my book.

As for your issue eswarden, these stepper motors are know be be finicky but can you confirm that you have the OEM recommended tire size or that the computer hasn't been tuned to change that?

Click to expand...

the sticker on your driver door says the 265 were the stock size?

eswarden said:

Tire size is larger than stock and the ECM has been configured by Blackbear for the tire change. However, the speedometer reading was off long before any of that took place. It's weird because if I'm doing 20 it reads 15, but when I'm doing 60 it reads 70.

Click to expand...

what about gear ration in pcm?

It's a 3.73 actual and in the PCM.

Sent from my LG-V410 using Tapatalk

have you extracted the mph data as seen by the pcm itself before signal reaches cluster?