Thanks for the replies.  I had a bunch of Y block Fords back in the day and the weak link was always the three speed tranny.  It always seemed to kill second gear at the worst time.   Of course I drove like an idiot back then, so it's no wonder.   I will not be drag racing this TBird, just driving around town.  I do plan to hop up the 292 a little.  I definitely don't want to cut the floor of this car.  It's so perfect, God would smite me if I put a saw to it.   I'm going to pull the motor and transmission soon and I'll check the O/D to see if it has a problem.  If not, I'll probably seal it up and install an electric dash switch so I can switch it into O/D on the freeway.    I am aware that starting out in O/D will kill the sun gear.

I ran a T85 overdrive in my '57 Bird with the E carbs for a lot of years. The big advantage was that I could run a 4.10 rear end and still drive it on the highway. Floor it and hit the kick down switch at 65 mph and when it shifted down the dual quads opened up and all hell broke loose. Back of of it at 6 grand and shift into overdrive and keep going. Lots of fun. The toploader 4 speed I am runing now is more fun but sometimes I miss the overdrive.

In 1968 I pulled a Fordo and put a T-85 O/D in a 55 bird. I didn't know it couldn't be done because of the smaller tunnel. It's very tight, and I think I had to cut a small notch in the top rail of the X member on one side. But only like 3/4". I used a Hurst synchro lok shifter, and that required some bracket changed to raise it up, and a pretty good cut in the top of the tunnel. The standard Hurst rectangular boot covered it ok.

If you're thinking of an O/D transmission as a 4 speed, I can see where you're disappointed. Think of it more as a 2 speed rear end. That said in today's world I'd replace the O/D with a T-5, which I did in my current 55 bird. I happen to like close ratios, and used the 2.95(?) low and the 0.80 5th. Most people use a higher 5th (I think it's like 0.67), and some the 3.27 or so 1st gear. Mummert has an adaptor that makes it easy, and brings the shifter out pretty close to stock.

Unless you put big tires on it and a lot more power, and hammer the hell out of it, the T-86 with the R-10 O/D will be fine. They went millions of miles behind y blocks and other engines. One thing that won't change with a T-85 is moving that heavy first gear when you shift. It's not like hitting second in a T-10 or other more modern transmissions that replaced it.

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

Color me wrong...

I new of a person that put a t-85 in his 55 and found out the hard way that the transmission hump/tunnel is larger on the 56/57 Bird. He had to alter the floor pan to make it work. You may want to check this out for yourself.

"One more question please.  My '55 TBird has a top loader T-86 3 speed tranny.  I understand this to be a light duty transmission.  Does anyone here know if a T85 will swap into my car in place of the T86?  If so, do I need a TBird specific transmission?"

I'm certain a T-85 will swap into your Bird, the '56 Birds had 'em and maybe some '55s. What I'm not certain of is if it needs to be specific to a Bird. I'm sure someone here will know, or you might contact Mac Van Pelt at Van Pelt sales. He'll know and he may even have one for sale.

[quote][b]Florida_Phil (9/17/2017) The quick acceleration and engine braking you get with a manual transmission are much of the fun.

I agree. I especially like to wind it up in 2nd OD as I approach a wooded cove, and then listen to the glass pack music when I let off the throttle. But, you may discover very little 'fun' in having to replace worn syncros, throw-out bearings, etc. The best features of OD (aside from reduced wear & improved gas mileage) are instant kick-down acceleration without shifting, and the ability to avoid lugging the engine by pulling out of intersection turns in a lower gear after merely punching the throttle.

6 VOLTS/POS. GRD. NW INDIANA

My assumed hype of the Ford O/D comes from the fact that many high performance Ford cars previous to 1962 were O/D equipped.  This makes sense as the low axle ratio would have raised their performance.  I see now that there were other reasons for purchasing an O/D car other than performance.  It never dawned on me that someone would care about gas mileage or brake wear on a hot flat head, 312 or 352.   Not having to use the clutch to shift seemed even less of a feature.  Having a low axle ration around town with an O/D function for highway driving is a very cool feature I intend to take advantage of.

One more question please.  My '55 TBird has a top loader T-86 3 speed tranny.  I understand this to be a light duty transmission.  Does anyone here know if a T85 will swap into my car in place of the T86?  If so, do I need a TBird specific transmission?

That is the performance Ford and the other brands that offered OD advertised, better gas mileage and less wear. Also the ability to shift without touching the clutch as long as you're below the governor cut in speed, generally 28 mph. I can see how the kickdown feature could be a little disappointing to someone who's used to driving an automatic. The OD kickdown doesn't have that "kick in the a$$" feeling because the OD is 30%, give or take. An automatic shifts down to 2nd or even low on kickdown. They get away with it because the axle ratios are tall. OD equipped cars have axle ratios ranging from 3:90 to 4:11 or even 4:56 for wagons. 

I'm not sure what performance hype you are referring to or what you expect? I've used OD trans in almost every early car I have owned since back in the 60's. My current 57 Ranchero and my 60 F100 have OD trans that I installed. The only performance advantage I know of is the ability to run a lower rear gear for around town driving and then using the OD on the hiway to lower rpms.