Speedometer inaccurate

[cc333]

The speedometer in my '94 was off too. I got another (with tachometer; the original cluster was speedometer only) and it, too was off.

My solution was to actually pull the needle off while going at some reference speed (30 MPH, in my case) and carefully push the needle back on after aiming it at the right mark. As you can imagine, it wasn't easy, and I didn't get it quite right, but better than before (it was reading maybe 3-5 MPH slow (would indicate 25 when actual speed was 30 ish), now it's more like 1-1.5 MPH off).

If there's an easier way, I will be keenly watching this thread to find out.

c

 

[Sean Buick 76]

I’m betting the rear axle ratio is different than stock.

 

[Erik the Awful]

Is the speedometer always 15-20 mph high, or is it a few mph high per 10 mph of speed?

If the speedometer is always the same amount high, pull the needle and put it back on in the correct location. If it's off at a rising rate, your tire size is smaller than it's supposed to be or your axle ratio is higher than stock. I put a set of 4.10s in my Mustang last year and now the speedometer reads 25% higher than my actual speed. That's 2.5 mph high for every 10 mph, and at 80 mph it says I'm doing 100 mph.

 

[The_Family_Tahoe]

Let me start by saying I did a search for similar issues and found several but not a fix. I have a 1996 GMC Sierra 1500 5.7 2WD and my speedometer reads 15 to 20 miles higher than actual speed with a scanner I read live data and the computer is seeing the correct speed from the VSS. I had the instrument cluster checked and was told the speedometer is accurate, could the problem be the computer? The wiring schematic shows the computer sending the signal to the instrument cluster.

I have the same issue with my 1999 Tahoe 5.7.
I've tried other instrument clusters and VSSs and get the same result. Stock 3.42 axle ratio. Vehicle instrument shows roughly 10% faster than actual speed. OBDII scanner reads correct speed verified with GPS indicated speed.
Interested in a solution or testing recommendations to further isolate the issue.