Or should I say dreamometer lol. I knew the speedometer was off a bit the first time I pegged it on the pin showing 130mph, so today I decided to check it with my GPS just to see how far it was really off. Found out when the speedo reaches 130mph, my gps is only reading 95mph lol. Even worse, I kept the throttle burried to the floor for close to 3 miles on a long flat straight stretch of road, and still only made it up to 107mph in 5th gear, this was at 4100' elevation. I think if I was closer to sea level I might be able to squeek out 115mph but that'd be about it. Guess it's either time to start modifying it, or sell it once spring gets here, and start building something faster lol.

 

Yeah, my speedometer is off too, I'd say its around 4-5 mph too slow.

Like if it reads 45, I'm actually going 40.

 

I'm reading 80 when I'm only going 65.

 

Which car is this?
Your 1979 car should have the 130 speedo unless you've swapped that into your 1982 car.

The '82 Z should go to 100mpg in 4th gear, then shift.
If you have changed out the differential then you need to have the matching speedo cog installed in the side of the transmission to make your speedo read correctly.

 

This was in my '82. The only thing I have done to the car since I got it was pull out what I assumed was the stock 3.9 R200 and replaced it with the R200 (which should have been a 3.9 also) that was in my 2+2 parts car because the old rear had a bad whine to it. The 130mph speedo was already in it when I got the car. Yes I did hit right at 95-100mph in 4th gear, but it didn't have enough power to pull past around 4400-4500rpm in 5th, hense the 107mph top speed lol. Once the weather warms up I'm going to swap over the speedo drive gear from the 2+2, hopefully the one in there matches the diff gearing lol. I'm also going to be putting on some 225/60/14's in the rear and moving the current 215's up to the front, so the bigger rear tires should help the speedo read closer to what it did stock stock.

 

hmm... 2+2 1980 5-speed 'might' have a 3.7 R200

or maybe someone swapped in a 3.54 R200 before you got the parts car. You never know.

 

If that was the case, swapping from the 3.9 to the taller 3.7 or 3.54 would cause the speedo to read the other way.

 

If that was the case, swapping from the 3.9 to the taller 3.7 or 3.54 would cause the speedo to read the other way. Either way something's up with the speedo gear not being correct lol.

 

how accurate is that GPS?

 

When I first started getting my '82 on the road. I put the tach past 5k, and my 130 speedo was buried. (roughly 5300-5400rpms) I know I was flying, not sure quite how fast though.

 

The GPS is very accurate compaired to most automotive speedometers. It takes location readings every 1-1 1/2 seconds, with accuracy up to as close as 3' depending on how many satellites are locked in at the time. I know it's dead on with what those roadside speed radar's display when I drive past them.