I like the 3.73's just fine. But I'm running a 26+" tire, a little shorter than stock, and the .8 O/D. Stock tires and the .6 something, I'm not so sure I'd be ok. You seldom run over 70 in western WA, the limit is mostly 60, and there's a lot of hills.

Edit-I should add I'm running a T-5, with the 2.9? 1st gear. Bit more than a T-85 1st.

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

You have to consider weight, tire size etc.
57 Ranchero 292, 27" tires with me, wife and full tank of gas weighs 3800lbs  3.89 works perfect.
60 F100 Panel 223 six 31" tires with me, wife and a full tank weighs 4500 lbs 3:89 gears are actually to high and I wish I had put 4.11. The 223 doesn't reach a good torque range until I'm doing 70mph, below that it is a dog..

My ’55 Customline with its 272 originally came with a 3.89 gear with the T86 overdrive.  I now run a 3.56 rear gear in a 9” housing with the T85 overdrive and it’s very comfortable at 70-75 mph on the Interstates.  At 125K+ miles on the engine since it was last into, it still pulls 16-18 mpg on a trip.  A heavier vehicle would probably be happier with the 3.89 gears though.

Lorena, Texas (South of Waco)

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


"FREE SAMPLE"

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..

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

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

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

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

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