JDJP

iTrader: (2)

Ok so my car has the GU2 code on the sticker, so it came form the factory with 2.73s. The former owner must have changed them to either 3.23s or 3.73s, from what I can gather looking at the rpms.

The speedometer has been off since i got the car, but not the way it should be, from the posts ive read. By having a friend drive next to me I have found out, when my speedo says 30 im going 37, and at 65 im really going 80, so it gets worse the fatser i go.

If it was because of the gears, wouldn't the speedo say i was going FASTER than I really am?

I told the guy at the shop and he says it sounds like a bad speed censor?

What do you guys think?

Thanks
JD

2001NBMZ28

Since you suspect the speedo isn't reading correctly, have a buddy to 70 next to you on the highway and see what your RPM's are, then compare it to a calculator like www.f-body.org/gears/

waveoff

iTrader: (11)

Your assumption is correct...it sounds like you have 3.23's in there. My car did the same thing as your car when I first got it. Take a look at this thread I started https://ls1tech.com/forums/general-maintenance-repairs/351727-speedometer-way-off.html

2002_Z28_Six_Speed

iTrader: (19)

Sounds like someone didn't recalibrate the speedo correctly? I think the non-linearity would go away if you corrected the info in the computer. The speed sensor is not bad.

waveoff

iTrader: (11)

Find a used HPP3 for cheap and use that to fix your speedo issue.

urbanhunter44

iTrader: (1)

If you gear lower, the speedometer reads higher, not lower. I installed 4.10s in my old car that originally had 3.23s. It read 85 mph when i was going 65 mph.

JDJP

iTrader: (2)

actually from using the gear chart, with the RPMs I know I have 3.73s
Ill start looking for a HPP3

waveoff

iTrader: (11)

That gear chart seemed like it was off for me. At 70 mph I'm showing 2300 to 2400 rpm's (A4). According to that chart, I have 3.73's also...but I KNOW that I really have 3.23's.

To be sure, why don't you jack up the back end of your car, make a mark on your driveshaft, turn a drive wheel one full turn and count how many times the mark on your driveshaft comes around...that's what I did. I also read somwhere on here about how you can downshift into 3rd gear (A4) and run the car at 75 mph (I forget the exact speed)...whatever the RPM's are is what gears you have. For example, if you were showing around 3200 RPM's that would mean you have 3.23's installed, 3700 RPM's would mean 3.73's, etc. I used both of these methods when my speedo was off and it worked for me.

Give these methods a try and let us know what you come up with. However, you might want to do a search on how to do each method. With the jacking up and wheel turning method I'm pretty sure you put the shifter in neutral but I can't remember for sure. And with the downshift/RPM reading method I can't remember what speed you have to maintain.