Axle ratio question
 

Hello all I recently purchased a 1970 gto with the original M21 trans, the previous owner said the axle ratio was 3:55 from what I have been reading the car would have been equipped with a 3:90 rear end. I know its not a 4:33 from driving it, what are the chances that it is a 3:55 ratio, the axle code is no longer visible on the tube after 40+ years of rust and paint. Thanks for the help.

I think PHS will answer this but, yes, M21 normally meant 3.90 or lower.

If it was a 455 car, i think 3.31 would be the standard gear and 3.55 optional. Does it have a 12 bolt rear?

Jack up the rear and secure it.
Rotate a valve stem one 360* rotation.
Count the number of rotations the driveshaft yoke makes.

On a posi - it will be the full count.
Non-Posi - double the count. (1.75 turns would = 3.50)

factory A/C car? no 3.90's with A/C.
how do you know it's a M21 and not a M20?

The car has a 400 CI. engine, the window sticker has G80 safe- t- track differential- HD 361, underneath that there is G92 axle-special order 362 but I haven't found what the codes translate to.

No air, the trans is coded to a M21 which is on the window sticker and the serial numbered is stamped on the trans which matches the car.

On a WT or WS car , those codes can apply to 3.90 or 4.33
The factory build sheet would be the only way to know which of those it was built with.

You would probably recognize a 3.90 while driving it.
Previous owner was probably right about a 3.55 being under it now.

the car is 44 yrs old, lots can happen in 44yrs. jack it up,pull the diff cover count and devide the teeth, you will then know for sure...no guessing.

There you go. No guessing.

Thanks for everyone's input, I did the tire rotation test and it is indeed a 3:90 gear ratio.

What motor do you have?

The motor is a 400, WT coded.

That motor is rather rare with the M21.

that's great!

a 400, m21, 3.90 is a great combo for spirited driving. enjoy.

how about some pics to share?

Another thing you could check, for the HD rear, is the casting number on the back of the rear, which should be 9799202. There will also be an "N" cast into the top front.

The casting number on the axle looks like 9799202; the number's are well worm do to corrosion over the years. I'll try to get some pics later this weekend, I'll have to read up on uploading pics the to the web sight.

Here's a few pics.

Smart way to order a standard GTO.
Poor boy way of getting max performance on the cheap.

very nice!