Now you've got ME confused! I have 245/50ZR- 16's on my 2001 Z28. If the GU2 means I have a 2.73 rear gear ratio, then how did I do 142mph in it? Ah heck... now I'm wondering if I need to reprogram my PowerProgrammer from a 2.73 to a 3.23 rear?

Ugh!!!
DS01Z28

 

The 2.73 gears will give the car a higher top speed than 3.23 gears.

If the sticker on the door says GU2, then you have 2.73 gears.

 

Will A4's with 3.23's not hit ~160mph?? (or anything over 150mph?? ). I know my car (with 2.73's) was still in 3rd gear above 150mph, but will cars with 3.23's not hit that speed in overdrive??

And DS01Z28..............there's nothing wrong with your car! Since you've got Z-rated tires (from the factory) you don't have the low speed limiter, therefore you can do over 115mph, and upto ~163mph if you're brave enough!! Pretty much the only time 2.73's are good is out on the highway for top-speed runs.