Rayzzz

What is the stock rear gearing on my 05 C6 A4 convertible. It has the mag ride. F55 I believe. Looking at rear end gear change.
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Rayzzz

Also how much will I need to pay to get a rear end set up with 3:90 gears. Who should I purchase one from.
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chsmith112

2.73 it is pretty hard to find a new diff for an 05 A4. But there many places that can make one for you but it's not cheap.

Rayzzz

Is there a rear diff out of a vette that has lower gearing that I can just swap in to my car.
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keyplyr

I recommend 3.73 (not 3.90) Much more manageable for street as well as huge pull on track.

http://mysite.verizon.net/foofoo2/c6/A4-3.73.html

I did the whole rear end: 3.73 gears, 2800 stall converter (flash to 3600), FLT Stage5 A4 trans w/ cooler.

Your local Corvette motorsports shop should be able to fix you up

LS1LT1

That is likely the gears he has unless the car was originally ordered with the axle ratio option then it would have the 3.15s (which was also the gears included on automatic Z51 cars), this info applies to 2005 cars only of course.
Only way to know for sure is to either look at the build code sheet or the rear differential itself.

quick Ag

The easiest way to find out what rear end ratio you have is to look up the RPO codes located inside your glove box. Look for one of these two codes: GU4 = 2.73 rear ratio, G90 = 3.15 rear ratio. I had 3.73's installed in my 05 A4 and they are awesome. Cost me about 1500.00 to have the job done.

Good Luck

chsmith112

What rpm are you 3.73 guys running when you care cruising at say 70mph?

keyplyr

The chart I linked above says 2300-2400.