My speedo isnt reading right, why?

[Blue Streak]

On my 96 GTS with stock everything (tires,rims,etc.) my speedometer doesnt read correctly. I noticed this last night running beside my friend Cobra. He was showing 70, when I was showing about 63. To futher back this up with proof, at the KY state line they have a portable radar set up to show your speed. I was the only one on the road last night when I went through it. I set my car on 50 and wouldnt you know, the radar showed 55. Can anybody help with a remedy to this problem? I hate that it is off also because it will read alot of miles that wasnt traveled.

 

[ViperRichRT10]

Have you had the car since new? Maybe you don't have stock gears and don't have a speedo recalibration box???????

 

[99 R/T 10]

Sounds like you have gear, 3.45 probably. Check the rear end and look for a speedo calibration box. It sould be close to the PCM on the left side of the engine compartment.

 

[Ulysses]

If you had stock tires and a 3.35 or a 3.45 or anything over the stock 3.07, you would be reading faster MPH on your gauge.

1) your tires are actually ~10% larger (in circumference) than stock
2) your sensor, or PCM, or gauge is off.
3) you have, for some odd reason, a higher gear (lower in number than 3.07)
4) Perhaps you had gears and a calib. box and the gears were replaced with stock but the box was not removed.

 

[Blue Streak]

I think it is the stock gear. What RPM's does it turn with a 3.07 in 6th @60 MPH? The tires a Psports(275 in front and 335 in back). Not 100% sure about the calibration box. Not for sure how to tell if it had one if I seen it. Are they normally black? Anybody else?

 

[joe117]

When I had stock gears, I don't think I could put the car in 6th at 60mph.

Anyway,
As others have said, you probably have the cal box but the gears have neen removed and returned to stock.

Or perhaps you have non stock gears and the box was set up wrong.
Example would be, you have 3.45 gears that someone put in but they set up the box for a 3.73

 

[Ulysses]

What size are your rims?

 

[Blue Streak]

My rims are the factory 17's. I looked tonight and didnt see anything around the PCM that didnt look factory.

 

[Ulysses]

One thing you might try is have a friend with a obd scanner take a ride with you. Hook up the scanner then take it for a spin and compare what the PCM says to the gauge.