I bought the car used, I have a feeling it's geared but I don't have any evidence to know for sure. Here are the reasons.

1) I drove by a radar station, while it said 50 on my spedo I was actually going 36. When I was doing 60 on my spedo I was really doing 43. I didn't keep going for obvious reasons. The speed difference goes up exponentially.

2) At highway speeds, my rpms hover at around 3k. On my spedo typically at highway speeds it says 95-105 but I just keep up with traffic so I can only assume I am doing around 65-75 possibly 80-85.

3) When I bought the car there was some grease on the differential, indicating a leak or it was opened. I have had it for about a month now and there's never a drop on my driveway and it looks like it doesn't leak.

So do you guys think my rearend was geared by the previous owner? If so what gears to you believe he put in?

 

by what you explain, it sounds to me like you have different gears. not sure, but my guess would be 4.10's

 

Its not going up "exponentially".... its going up as a fixed percentage. 50/36 = 1.389; 60/43 = 1.395; 100/73 = 1.370. You have about a 36-39% speedo error.

If your car started with 2.73 gears (GU2 on the RPO label), and someone installed 3.73's, your would have a 36.6% error. Seems the likely explanation. Look for the rear axle ratio code on the RPO label. Then get under the car, and turn the rear wheels around once and see how many times the driveshaft goes around. See if it checks out.

 

Well at highway speeds what rpms should I be expecting from a stock rearend in an A4? Highway speed = 65-75

 

Depends on what your stock rear axle ratio is/was..... could have been either 2.73 or 3.23. That's why you need to get the "GU*" code from the RPO label.

At 70MPH, stock tire diameter, in O/D:

-2.73 - 1,750 rpm
-3.23 - 2,070 rpm
-3.73 - 2,390 rpm
-4.10 - 2,630 rpm

To reach 3,000 rpm in O/D, you would need to be going:

-3.73 - 88 mph
-4.11 - 80 mph

 

Well judging by those numbers I more then likely have 3.73's in the rear end. I took my z28 to the local tranmission shop to have a tranny cooler put in and while I was there I asked about the rearend, they confirmed that I was geared and they will readjust my spedo for me.

Thanks for the help to everyone who replied, cz28 is a great community. I've only been around for a few weeks but I wish I had found you guys sooner. LS1Tech didn't have enough info on the lt1's but here is great.

 

i have a 94 z28 that i recently bought and i'm not sure of the gears. I did a rought figure on it and came up with 4.28 . And this is how i got that.
Ok i have a six speed car and fourth gear is 1:1 right, so in fourth gear i can go 50 at around 3000 rpm. my tire diameter is 24.5 which comes up to 6.28 feet around and at 50 miles per hour is roughly turning at 700 rpm. Initial rpm is 3000 divided by 700 output is 4.28. Am i doing this right?

Is it safe to assume that i have 4:10 gears in my rear end?

 

Sounds like your tire diameter is off. Stock tire diameter is 25.6". The rolling radius is closer to 12.6"

Based on my 94 with a T56, and a spreadsheet I built for MPH/RPM as a function of various tire diameters and rear axle ratios, in 4th gear with stock tire diameter:

3.42: 65.8mph
3.73: 60.3mph
4.10: 54.9mph