Inaccurate speedo?

[Akodo]

Drove the T/A to work for the first time today. Yee-haw. Forgot how much I missed the rumble and purr of a real car. Didnt miss the bone-jarring shakes from the Michigan potholes, though.
Gotta learn how to dodge better.

So my problem: Either my Speedo was reading horribly wrong, or everyone was in a BIG hurry on 96 this morning. While "pacing" the slower traffic, my speedo was reading about 98mph. Rather doubtful.

So where to start? Its a digital dash, I'm assuming its getting its info from the ECU, which is getting a reading from a sensor somewhere, probably on the driveshaft? The rims are stock, though I don't think offsize tires would cause a 20 mph swing anyway.

The Tac isnt reading either, could that be playing into the problem?

[MP81]

I know the electronic speedos have a sensor in the transmission. Vehicle Speed Sensor, or VSS.

[meissen]

Overall wheel height - rim and tire combined - will change your speedo but not 20mph unless you put mudding tires on there or something. When I bought new rims and tires on my 6 it shifted the speedo off by about ~5 mph. Interestingly, when I changed my dash from 93-96 to the 97+ dash - the 97+ speedometer automatically calibrated itself to the correct speed limit.

Might be a combination of the offsize tires with something else being wrong.

[MunsterZ28]

did you have the rear end gears changed?

[98blackbeauty]

For the speedo to read 20+ fast you would have had to put a high gear rear end like 2.50 ratio. of so. I would think you have maybe a vss. gone bad but maybe just binding from sitting all winter. The Vss is located on the drivers side by the gear shift lever.

[Akodo]

did you have the rear end gears changed?

Nope, far as I know, theyre stock. I'll check out the VSS. Its a manual trans, is it still located in the same spot? Should be interesting to get to.

[UjGearhead]

One way to see if it's the ECM or VSS is find somebody who has a decent OBD1 scan tool. It will tell you what speed the computer THINKS it's going. If it seems about right then you probably have a problem with your cluster.