When I got my car, I had the RPO code GU5, aka 3.23 gear ratio if I am correct. Shortly there after my rearend started to howl, clank, bang, etc. so we decided to buy another used rearend. My dad found one, since I was in high school at the time, and brought it home. It was a complete setup, ie: housing, axles, brakes, lines, everything except tires. We were told it was from a 95 z28 i think but I can't remember. I'm pretty sure it was from a z28 b/c I used to be able to lay this http://memimage.cardomain.net/member...6_122_full.jpg
But now I can't and my rear is howling bad again. Everyonce in a while I get both tires going again but mainly its just the passenger side turning over. But that's another topic . But back on topic. After I got the other rear in I noticed that my speedometer was showing 5 mph over what I was actually running. Example: When I'm on the highway, I run an actual speed of 85 mph, but instead my speedometer shows 90 mph. Just from my general knowledge of gear ratios, which is not much I assure you lol, I would need to step up in ratio for my speed to decline in my final gear which is 5th. The car is a 94 3.4L 5spd. I've tired to calculate my gear ratio by using the RPM and MPH calculation on this page https://www.camaroz28.com/services/calcs/
Maybe I'm doing it wrong but I cannot figure it out because the speeds and rpm's it show's are not matching up to what the car is actually running. I've been clocked at the above scenario so I know its correct. So if anyone can figure this out for my I would be ever so grateful. Here's my specs:
Final gear ratio: IDK, but I believe it might be 3.73 to 1
Tranny ratio: .74 to 1 in 5th gear if I'm not mistaken
Tire Diameter: 25.64 inches (245/50/R16)
Engine RPM: 3000 rpm
Vehicle speed: 85 mph (real speed)

When I plug this in I get 83 mph but i'm at 85 so maybe I have 3.73's. Possibly the calculation has some error in it? sorry for the long post and if this is in the wrong forum mod's plz move it. thanx again, l8tr

 

is it possible the rear end is from a manual transmission car? (they come with 3.42's, largest offered from factory on f-bodies)

 

Sounds like you've got 3.42's in there .

5 mph / 85 mph = 5.8% difference
3.42 - 3.23 = 0.19 / 3.23 = 5.8% difference

...coincidence??? Methinks not!!

Seems you got a diff out of a '95 Z28 M6 (all 6-speeds came with 3.42's).