Gears & Axles - Mystery Gears!

Well I blew the 10 bolt up awhile back and bought a whole takeout rearend from a local shop. It is suppose to be a 3.42 out of a M6 Camaro. I had my CPU reflashed for shorter tires last weekend and I seemed to be passing everything on the road. So I broke out my handheld GPS and set it up on MPH. The car is at 1800rpms and the speedometer says were running 50 mph. But the GPS says 70mph. Whats back there now 2.73s?
I looked at the sticky and they only list 3.42 thru 4.10s gears for an A4. Where can I find the 2.73 and 3.23 gear charts for a 4l60e? Stock A4 cars whats your MPH at 1800 lockedup?

V-6, 3.23 a4 is 2000 at 70.

Pull the cover, count the teeth and divide the ring gear by the pinion gear.

Why the hell would you pull the cover when all you have to do is count the number of turns of the driveshaft?

Why the hell would you pull the cover when all you have to do is count the number of turns of the driveshaft?


b/c the oil needs to be changed

Jawsgear.com has some very cool tools:
http://www.jawsgear.com/calc_rpm.html

I put in a gear ratio of 2.73, tire height of 25.66 (275/40r17), speed of 70mph, OD gear ratio of 0.75 (not sure, maybe it's 0.7?) = 1876rpm