Picked up a 93 Z28 ( A4 ) that originally had 3.23s in the rear ( GU5 RPO code ) but the speedometer is consistently 10-25mph ahead of what I'm going.

Originally I just thought it needed to be fixed but after driving it on the highway, speedo read ~2200RPM at ~60-65mph ( speedo read 90mph )

My 88 GTA ( also an A4 ) I had previously had 3.27s and was dead on 2kRPM @ 65mph.

I'm assuming these would be something like 3.42s, somewhere around there.

What are your guys's thoughts.

 

Gu5 is a 3.23 gear if know ones changed and if it has g80 behind it limited slip posi

 

The car came with 3.23s

It does not have those anymore.

 

sorry got ahead of my self the easiest way to find out is to pull cover and see whats stamped on ring gear

 

Will do when I get a chance. PO said he thinks they're 3.77s ( owner before him did all the mods and they know each other )

 

Mathematically, if your car had 2.73 (GU2) from the factory, and swapped in a set of 3.73 gears (not 3.77) the speedo would read 36.6% higher than actual speed. At a true 65 MPH the speed would read 89 MPH.

 

This might help you calculate, but you could just lift the rear, spin the tire one revolution and count how many times the driveshaft turns. The amount of turns should equate to one of the standard available ratios (ex. 3.23. 3.42, 3.73...).

 

Only problem with that theory Injuneer is that the car had 3.23s from the factory. Gu5 RPO is present.

 

Update: I did as you suggested shoebox and spun the tire. The driveshaft only spun around ~3 1/4 times which would mean 3.23 gears...

The RPO code confirms the car had 3.23s from factory so why the heck is my speedo so inaccurate and the RPMs higher than they should be as well?

Only thing I could think of that might affect it would possibly be the timing?

 

Maybe someone has played with the driven gear for the speedometer or changed transmissions. 93s did not have a VSS like 94 and up cars did.

 

Both possibilities as I'm not the original owner nor is the owner before me.

Since you mentioned transmissions, would a shift kit affect the speedo at all? It would make sense that one of the POs had a blown trans, got a new one and put a shift kit in while he had the opportunity.

The front tires are are a tad smaller than the back but I don't think the front has any bearing on the speedo.

Another question. Since the speedo is so off, I'm assuming its affecting the Odometer as well. Correct?

 

Shift kit will not affect speedo.

Vehicle speed sensor runs off the tailshaft of the trans so only rear wheel outside diameter affects VSS output, not the front wheels.

If your speedo is reading 37% higher than actual speed, every time you drive 1,000 miles the odometer increases by 1,370 miles.

 

try this sight and plug in the numbers:

http://www.f-body.org/gears/

 

2200rpm at 65mph, assuming a stock 0.70:1 overdrive in the transmission and stock 245/50R16 tires, corresponds pretty exactly to a 3.73:1 rear end gear ratio.

Before you worry about transmission modifications, you should make absolutely sure that you counted your tire rotations and your driveshaft rotations correctly.

Another possibility is that your tachometer is off. I've heard that it's common for them to read high, and that would skew the math.