Ok, not sure this fits in here...but it's the best I could figure...

Little background, I'm getting parts for my dad's 50 four door chevy, this thing has a 55-64 passenger car rear end....problem is it's got some STEEP gears in there. So I'm looking for some highway gears, comperable to 273's.

So am I out of luck, are we going to put in an OD tranny or a different rear end, or are there suppliers for these types of gears for this rear end?

You still should be able to get gears. Try Summit, Jegs they should have them.

The highest ratio in the 55-64 style rearends was 3.08. The 3.08 was available 59-64. Although, it MAY have been in some 58s with a 348.
Until recently, NO ONE made any gears to fit these rears that was higher than 3.55. But recently, US GEAR has been manufacturing 3.08 and 3.36 gears. Also, US GEAR and Eaton have been manufacturing posi units for these rears. Prior to now, ONLY the original 57-64 posi units were the only thing available. Unfortunately, they have become rather scarce, expensive and, because of their age, it is common for them to have some cracks through the bolt holes where the 2 halves bolt together.
I think both the US GEAR and Eaton posi units are as good as the original GM units (made by DANA) were. But, compared to a 40yr old original, they are MUCH stronger because they are new and and haven't been stressed or abused. The same goes with their gears (US GEAR is the only one making the gears).

Ok then, the gearing in the tranny may be set wrong...

I know there is a formula for determining the ratio by turning the rim and watching the driveshaft...

Can someone tell me what that formula is?

That was some awesome info DZ...thanks

The ratio is determined by turning BOTH rear wheels ONE turn, and counting the number of turns made by the driveshaft.
So, Jack up both rear wheels, mark both rear wheels with a piece of chalk or whatever-----------IN THE SAME LOCATION (I make a mark on the brake drum and the backing plate). Make a mark on the driveshaft. Put the tranny in neutral. If the rear has a posi (likely it doesn't), then when you turn one rear wheel, the other will turn also. If it is a non-posi rear (likely it is), then you will need to turn both wheels at the same time (stretch out your arms). Turn both wheels ONE turn and count the number of turns the chalk mark makes on the drive shaft makes. If it makes just over 3 turns, it is 3.08; 3 1/3 turns, it is 3.36; 3 1/2 turns, it is 3.55 and so on.

An optional way of determining the ratio, is to jack up one wheel (that is if it is non-posi). Mark the wheel and driveshaft. Turn the wheel ONE turn. Count the number of turns made by the driveshaft. DIVIDE that number by 2 (with one wheel only turning, the driveshaft will turn twice as fast). For example, with one wheel up, and turned once, if the drive shaft turns about 7.4 turns (divided by 2) then it is a 3.70.

Between 55 and 64, the factory available raitos were: 3.08, 3.36, 3.55, 3.70, 4.11, 4.56. (a ONE year only was a 3.27 in the 56 Vette, you'll probably never run across one of those).

Awesome....thank you.....

Just a thought....when I broke my housing in my 60 El Camino, I needed a new one. I lucked out and found one in a junk yard. $75 later I just changed out the housing just to try it out after a thorough cleaning. After a short test drive, it worked so well, I just left it as is. New rear for less tha $100! I re-installed my posi later on. Most cars were geared with 3.36's.