Speedometer reading MUCH too high..

[WingingEd]

I recently acquired a '96 Silverado z71 4wd and the speedometer is reading at least double what it should. I have replaced the VSS and the instrument panel, but neither made any difference. If I understand correctly, this truck has no DRAC/VSSB module and the VSS signal reports directly to the PCM (please correct me if that's wrong).
I find a lot of search results for the opposite of mine, speedometer reading zero, but hardly nothing about one reading too high.
I know the previous owner installed an engine, but he didn't mention anything about the transmission or transfer case. If he did put a different transmission in, maybe from a newer model, could that be the problem?
What can I do next to trace down this problem?

Thanks.

 

[kennythewelder]

Tire size, and gear ratio will throw off a speed o , and it can be by a lot. If the PO had 37 inch tires, and pulled them off, and put some 285/70/16, that may be the issue. This can be fixed by doing a recalirbration for the size tires you are running now. I use a GPS on my phone to set mine, along with a programmer. They sale a speed o programmer just for this. Summit, Jegs have them, and Im sure ebay and amazon does too.

 

[WingingEd]

Interesting Kenny, so the previous owner might have had it re-calibrated for larger tires? Right now it has 265/75/16, which is exactly what the door sticker says.

 

[kennythewelder]

Yes, It is possible. I have recaled mine several times. Then I swapped my cluster for a 120 MPH cluster,and had to recal it again. I have a Jet programmer I bought several years ago. It will tune the truck, recal the speed o, do live data code reads, change shift points, firmness, reset the top speed limiter, rev limiter ECT, but they arnt cheap. Mine was maybe $325. A custom tune is more effective, but I can change things anytime I want to. A custom truck shop should be able to help you, or even a decant trans shop. Maybe $50 to $100, IDK. The 120 speedo is in my truck rite now, and is spot on the speed it shows. If it says your doing 60, then your doing 60 on GPS. When I had the 100 MPH cluster, ( that the truck came with new) it was off about 4 MPH. I recaled it to read rite, before I got the 120 cluster. I also set my top speed to max. 225 MPH, my truck does not cut off at100 MPH any more. It will never do 225 MPH, so it will never cut off again.

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[kennythewelder]

This is the programer I have. Just click the link. Since your speedo may have been recaled, you may just want a trans shop to redo it. I was a little dissapointed in the engine performance gains. I thought it would be more aggressive than it is, but it has been nice to have may times with the live data code reader. I did see an improvement over stock tune. The biggest improvement was trans shift firmness. It shifts chrisp now. No more long slow shifts. I did not fool with shift points at all. I just set mine to max performance. I have to run supper unleaded.
https://www.summitracing.com/parts/jet-15024/

 

[WingingEd]

The instrument cluster I swapped in is from a 1998 model, and exactly like the one that was in my truck. I wonder if the PO might have already swapped in a later model cluster and I would need to go back to a 1996 model to get one that is compatible since my truck has no DRAC/VSSB module?

 

[kennythewelder]

Maybe but I doubt it. My 120 cluster in my 97 Silverado, comes out of a 99 Denali. Although it was off some when I installed it, maybe 5 MPH. The speed info is read by the ECM. When I plug in my Jet programer, it will also read MPH on the display screen of the programer. This is what the VSS is giving the ECM and what the ECM is telling the speedo to read.

 

[WingingEd]

OK, a little more information now.... I just connected my little obd2 device that bluetooth connects to my smartphone. When speedometer reads 55mph, odb2 says it's actually going 40.

 

[kennythewelder]

15 MPH can be fixed easy. It just needs to be recaled.

 

[df2x4]

OK, a little more information now.... I just connected my little obd2 device that bluetooth connects to my smartphone. When speedometer reads 55mph, odb2 says it's actually going 40.

15 MPH can be fixed easy. It just needs to be recaled.

I don't know if that's what's going on here... My '97s speed as reported by the PCM via OBD2 port is generally very close (at least within 5MPH) to what the speedometer reads in both trucks. It sounds to me like someone may have pulled the cluster needles to switch bulbs and not put them back in exactly the right places.