stock 3.73s???
 

i blew the rear end in my a4 with 2.73s and came across a rear outa a 99 ss. the guy i bought it from was just parting out the car and didnt know much about it. the car looked painfully stock(cb,lcas,phb everything). he said it was all stock with 3.42s. when i picked it up it had the AAM diff. cover and the auburn unit im guessing (turn one wheel and the other goes in the same direction) after i had the rear installed obviously my speedo was waaay off so i ussed my HPP3 to adjust my speedo and a GPS to verify. set it to 3.42s and my tire height and it was still off a little (40mph was really 35 and 90 was really like 77mph) so i set it to 3.73 and its dead on! i thought m6 cars only came with 3.42s? also anyone know any info about the auburn unit? any better than the stock torson? ive heard its actually weaker.

Well, normally the M6's do come with the 3.42's. The code is GU6=3.42. It's possible the previous owner might have got the wrong gears thinking they were 3.42's. I dont know it's hard to say. Either way, going from 2.73 to 3.73 is a huge difference.

Stock M6 came with 3.42

what about the auburn diff.? i've read it was an slp option but cant find any real info about it. did they offer any other gear ratios? i pretty sure this rear end was never rebuilt or gears swapped

anyone?

I've never heard of a 4th gen f body coming out of the factory with 3.73's
It wasn't an option.
As for the Auburn vs Torsen. The torsen will wear longer, the auburn will wear out one day.
I'd prefer the auburn, it's not torque biasing like the torsen. The auburn relies on seprating force in the side gears and spider gears. This pushes the side gears into the case and locks it all together.

3.73 = Never Stock
Auburn Positraction > Torsen