Cluster speedo not accurate

[densobro]

Speedometer has always been of since I got the truck.
(1996 GMC Sierra sle 5.7 4x4)
Had my wife drive next to me while on the phone with her to confirm my speeds.
When the Spedo reads 90, I’m going 70.
When it reads 80, I’m going 60.
And when it reads 70 in going 50.
I searched around for solutions and bought a refurbished cluster for the exact model (verified part #’s)
With the new cluster that actually had bulbs that worked I discovered tons more problems like air bag and abs braking.
But the new cluster read the same speeds. A found a company in Atlanta some what close to me that said they could solder the right pins for me to get the accurate readings, but doing that isn’t really fixing the problem.
A lot of stuff was done to the truck years before I bought and I wonder if the rear gears were swapped but not sure. The transfer case fuse is missing which makes me wonder. Has anyone else had this problem?
Black bear said they could send me a new computer with the right gearing programed but I feel like it would still be off.

 

[kennythewelder]

What size tires are you running. Gearing, and tire size will affect your speedo. Here is a grate video on how to find your gear ratio, without pulling the diff cover.

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[turboedge]

Is the odo correct? Fairly easy to check by by traveling a straight road between two roads a mile apart.

 

[sewlow]

Most OBDII handheld 'tuners' can recalculate the speedo once the gear ratio & tire size info is punched in.
They're stupid expensive for what they do. $300.00+
The tuners are VIN specific. Single-vehicle use.
But there's an easy way around that.
Got a bud that has one? Borrow his for an hour to set the parameters for the speedo. Plus what ever else you want done that the tuner will do.
Bring a dozen for the man-cave fridge.

 

[GoToGuy]

What tires did your truck have ? On the door jam or the RPO on the glove box. Start there first. Compare to what tires you have now. 20 is quit a bit off. Start with tire size first.

 

[PlayingWithTBI]

When the Spedo reads 90, I’m going 70.
When it reads 80, I’m going 60.
And when it reads 70 in going 50.

It looks like you're off the same amount at each speed, not the same percentage. If I had to guess, you're speedometer needle is in the wrong place - 20 MPH too high. i don't think gears and/or tires are the cause.

 

[densobro]

It looks like you're off the same amount at each speed, not the same percentage. If I had to guess, you're speedometer needle is in the wrong place - 20 MPH too high. i don't think gears and/or tires are the cause.

That’s what I thought but the refurbished cluster is the same speed, and it seems to be 20 mph off but when I’m driving 10 it says 12 and when I’m doing 30 it’s around 37

 

[stutaeng]

It shouldn't be off the same amount at each speed if speedo is working good. Example, assume it's off by 7% like if larger tires are installed:

@ 40 MPH : 40 x 1.07 = 42.8 MPH (difference is 2.8 MPH)
@ 80 MPH : 80 x 1.07 = 85.6 MPH (difference is 5.6 MPH)
...etc. it's a linear relationship

As you increase the speed, the difference should be increase also proportionally, and vice versa...

 

[densobro]

Is the odo correct? Fairly easy to check by by traveling a straight road between two roads a mile apart.

I don’t think the odometer is correct because after an oil change, 3,000 miles would come around a lot faster than with any other vehicle. I did zero the trip and drive to work which is around 35 miles away and once I got there it said I traveled 54ish miles

 

[densobro]

Most OBDII handheld 'tuners' can recalculate the speedo once the gear ratio & tire size info is punched in.
They're stupid expensive for what they do. $300.00+
The tuners are VIN specific. Single-vehicle use.
But there's an easy way around that.
Got a bud that has one? Borrow his for an hour to set the parameters for the speedo. Plus what ever else you want done that the tuner will do.
Bring a dozen for the man-cave fridge.

I started off by calling the local auto zone and seeing if their fancy hand helds could reprogram my gear ratio to try to get the Spedo close to accurate, but the said they don’t have anything that would go back that far (year model wise). They recommended some website that sells them, I think it was summit racing, so I called the local summit and they said they also don’t have anything that would go back that far. They recommended another place and same thing. So I called a local tuner shop to try to get the Spedo fixed (also a badass tune) and they told me they couldn’t tune anything older than a 1997. that’s when I found the cluster instrument repair shop in Atlanta, but I never pursued that. I was willing to buy a $500 hand held but could not find a single one that could tune mine guaranteed.