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]

Ok it's an error of percentage. That's why the amount of error is greater at higher speeds. 10% at 20= 22 or 18. 10% at 60= 66 or 54 . Clear as glass? So one way is blast down highway, call up smartphone GPS , or another GPS device. Drive steady speed, record indicated from tach and speedo, and what does ecm say. Against GPS speed. Result is how much and where errors are. That's what I did. Little larger tires 66 indicated is actually 70 ground speed checked by GPS.

K thanks I will try that after I get paid this coming weekend to get another obd2 reader as my old obd2 bluetooth reader has crapped out.

 

[Keeper]

a PO had the ecm re-programmed for different size tires?

 

[99xcss4]

a PO had the ecm re-programmed for different size tires?

this or gear ratio change

 

[Keeper]

OP should check that his RPO code is GT4, and not GT5

 

[dmhscorpion]

K thanks I will try that after I get paid this coming weekend to get another obd2 reader as my old obd2 bluetooth reader has crapped out.

My 99 Tahoe 2 door had the same issue. I bought this and changed the tire size with it.https://www.amazon.ca/VXDIAG-Multiple-TIS2WEB-Diagnostic-Programming/dp/B01NACTAY8/143-9213689-1581333?psc=

 

[Helrazzor]

Stock tire size should be like a 265/75/16. Think you have wrong size tires.

 

[Redix01]

Stock tire size should be like a 265/75/16. Think you have wrong size tires.

It's gt4 g80 its a C2500 and the door sticker says 10ply 245/75/r16. GPS while driving reads that my 55 mph is 64ish on the cluster the same speed that the pcm/ecm reads from the obd2 plug.

 

[Redix01]

OP should check that his RPO code is GT4, and not GT5

Gt4 g80

 

[Redix01]

So the tach and speedo indicated are both different from what you pull tach and Speedo from ecm?

Yes I had a bluetooth FIXD reader but it crapped out on me I took a screen shot while it was reading 55 on the app and rpms were reading 1765.6 but the cluster was reading like 1900

 

[Helrazzor]

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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