i bought a trans off a fellow board member a couple weeks ago and its out of a 93' and he said it came it 3.23s but seems too be right at home with the 2.73s that are in my car.. my speedo hasnt been reprogramed from the auto that was in it so i cant give you an acurate RPM vs. MPH too tell that way can i tell by the casting #s? what i THINK is about 80mph is around 1700-1900 RPMs.. i did have an MN6 in it and 6th was COMPLEATLY usless and now i can run in 6th down the interstate pretty well im just curious cause i had a 3.73 rear built that is going in, in the next day or two and im courious if its gonna be too short for my set up + i dont want to COMPLEATLY kill my gas miliage as i drive it around 100mi a day too work and back althought that is gonna compleatly stop in about a month as ill be moving across the street from the job

 

1. Punctuation makes a post MUCH easier to read and understand.
2. Why did you get rid of the MN6? FYI they are a lot stronger than the M28 and M29. The torque rating on the MN6 is 450 ft-lbs vice 350 ft-lbs on the M28 and M29. IMO, you could've stuck with the MN6 and just used 5th gear until you changed the rear gear.
3. Did the transmission come out of a '93 Formula or a Trans Am? According to my "Firebird Red Book", it says the Formula came with M28 and the Trans Am came with the M29.
4. 6th gear ratio in the M28/M29 is 0.62 versus the 0.50 in the MN6, which would explain the reason 6th seems more "useful" in the M28/M29. The final drive ratio for the M28/M29 with 2.73 gears is 1.69, which is about equivalent to an MN6 with 3.42 gears (1.71). The A4 you had in the car has an overdrive ratio of 0.70, which gives a final drive ratio of 1.91 with 2.73 gears.
5. If the transmission is the only thing you have changed, you can determine your speedometer error (in top gear) by taking the final drive ratio of the A4 divided by the final drive ratio of the M28/M29.
1.91/1.69 = 1.13
That means your speedometer is reading 13% LOW in high gear. IOW, when your speedometer says you're going 80mph, you're really going 90mph.
6. Oh yeah, that means it throws your odometer reading off that much too, so if you're using that to calculate your gas mileage, you need to factor in the error there too or your figures will be out to lunch.

 

i got rid of the mn6 cause the one i had was compleat junk, had a bent input shaft and i got an awsome deal on the other one. It was not my intention too go out and find an M28/29 thats just what it happend too be and i like it. The speedo is more than half off becaue it is still on the A4 program so its not even close enough too get an idea i think its about 55% off and it has nothing too do with the M6 gear ratios compaired too the A4. I know the differenced is ratios and torque ratings between the 3 trannys but without haveing a speedo that is useable i have no way of determining what gearing is in the trans i would also think i would need too know this so i can let whoever does my tunning when the time comes, and that wont be a for a wile as i have a few more mods too do before i spend the money for a tune im looking for some sort of way too determine the difference with the casting # or something.. BTW this trans did not have Cags from the factory eather if that makes a difference BTW M28=350ftlbs M29=400ftlbs and MN6=450ftlbs or so i understand

 

Just look up the casting numbers, unless the tranny was modified by someone else you should be able to find the factory gear ratios.

Once you have the proper ratios this will help you determine speed@rpm.

http://www.f-body.org/gears/