I've had my obs 1999 GMC C2500 suburban with l29 454 vortec 3.73 gears for 6 years, stock 245/75/16 tires the speedo read to high this is from ecm

[Redix01]

3 things will cause your issue. Wrong tire size which you have confirmed are correct. Wrong rear gear, not sure if you have confirmed that. Or the ecm does not have the correct ratio programmed into it. I have a 97 K1500 that is lifted with 35 inch tires. It had factory 3.73 gears that I changed to 4.10. I bought a Hypertech programmer that I could change tire and gear changes. Not sure if you can find a used one, but it might correct your issue.

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Not sure about the gearing yet but I did remember right after I bought it I had to have the rear brakes replaced drums and all. I had to wait a day extra bc the drums they ordered were slightly to small. Not sure if it has a whole different axle in it or what. Or they just goofed on ordering the wrong part. The dealer also gave me paperwork for instrument cluster repair but it says for a 98 Silverado 1500. But I will try and get

 

[Redix01]

Not sure about the gearing yet but I did remember right after I bought it I had to have the rear brakes replaced drums and all. I had to wait a day extra bc the drums they ordered were slightly to small. Not sure if it has a whole different axle in it or what. Or they just goofed on ordering the wrong part. The dealer also gave me paperwork for instrument cluster repair but it says for a 98 Silverado 1500. But I will try and get

A programmer as well

 

[Helrazzor]

Check your RPO codes in the glovebox. GT2=2.29. GT4=3.73. GT5=4.10. GT7=3.33. If it is a different GT code just do search for it.

 

[Redix01]

It's GT4

 

[Helrazzor]

Lift rear tires off the ground, put truck in neutral. Put a mark on drive shaft and rotate wheel one full turn and count the number of times driveshaft completes rotation. That will give you close approximation of what gear you have without opening rear cover.