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]

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[GoToGuy]

Ok this what ecm thinks is going on. But what is the problem? Pretend you showed just this page to your neighbor, and no other info, what would he answer?

 

[Redix01]

The instrument speedometer runs at 64ish and rpms run at 1900rpms

 

[Redix01]

Sry I had a picture that I took of the cluster at the same time I thought I still had

 

[Redix01]

Ok this what ecm thinks is going on. But what is the problem? Pretend you showed just this page to your neighbor, and no other info, what would he answer?

Sry still a little out of it from having back surgery earlier today, the cluster reads 64ish mph and the rpms 1900.

 

[k1500 97]

Sry still a little out of it from having back surgery earlier today, the cluster reads 64ish mph and the rpms 1900.

Dont worry man someone will chime in. I have 3.42s in mine i can run 80mph at little over 1500rpm. Thats me not hammering it steady cruising

 

[GoToGuy]

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

 

[Redix01]

Yeah I have to go 64ish to actually going 55 it's off all the way up but less at lower speeds not sure if someone put a different rear end gear in it before I had it or if someone had the whole cluster replaced with the wrong one? Would either one do something like that?

 

[k1500 97]

Im no help on that. Best of luck

 

[GoToGuy]

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.