What would be the optimum rear gear ratio for a 292 with a T85 OD? I'm going to go through my Dana 44 in my wagon while I am doing the engine swap. I finally found and bought the T85 OD I've been looking for so it is going in too.

Durham Missouri

3.89 to 4.11 were the usual axle ratios in OD cars. The OD effectively reduces those figures by 30%, so a 3.89 becomes a 2.72; a 4.11 becomes a 2.877. I wouldn't go any taller than a 3.89; you want to keep the engine RPM in the fat part of the torque curve when you're in OD on the highway.

In my 55 crown i ran 3.78 with od, in my 57 I had 3.70 , both cars came from the factory with these gears 

62bigwindow.  All good thoughts from others.  Several variables to consider...
- tire size
- engine cubic inches (engine torque)... a stock 292 has more basic grunt than a 272, but less than a 312. 
- engine camshaft (horsepower and torque curve)... stock or degree of performance (lift, duration, overlap, lobe center angle).
- engine compression ratio... makes a difference in pulling ability.
- intended use (type of driving... stop-and-go [city], terrain [hills or flat], highway [elevated rpm], vehicle weight, and loading expectations

The T-85s have a 2.49:1 1st gear ratio...
1st gear is not synchronized when down shifting...
The T-85 R-11 overdrives drop the final drive output ratio from 1:1 to 0.72:1 
Borg Warner intended the overdrive to only be used with the tranny in 3rd gear.  


NoShortcuts
a.k.a. Charlie Brown
near Syracuse, New York

In my mis spent youth, I put a Dana 44 with a 4:27-1 rear from a 6 cyl. O/D wagon. 312 motor, T-85 O/D, 27+" tire. In a 55 bird, so it was a little lighter. That was a nice ratio, had enough torque in 2nd to avoid having to double clutch to low all the time.

Dana ratios are a little different than 9". The other thing to watch, if you're thinking posi, the carrier changes at around 3.7 or so. I can't recall exactly, but one carrier for that down, and other for up.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Thanks guys. I think I will try the 3.89. My engine is warmed over with 9:1 comp and a mid range cam with mild porting on the heads with bigger valves. It should be around 275-290 hp. Most if not all the driving will be highway with some town driving. The tire size is 670-15. I plan on replacing everything including the axles and stepping up to a limited slip diff.

Durham Missouri

You might find something I missed, but what I found was 3.73, or 3.92. That's also the break point on the posi carrier. I've got the 3.73 in the car now, as I was too cheap to change the posi to do the 3.92. I had to go look it all up, can't trust the memory anymore.


miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

How well so you like the 3.73? Are you running overdrive?

Durham Missouri

Don't have a Dana so I used 3.89 in my 9". I live in hilly terrain so that gear works great and cruises on the hiway with ease when in OD..

You need a 4.11 or higher numerically in a wagon from my experience.


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