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By 62bigwindow - 6 Years Ago
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The rear axle in my car is going to need a rebuild. With a T85 OD what would be a good ratio to swap in? I'd like to lower the cruising rpm at 70-75 but not sacrifice too much low end. I was thinking maybe switching from the current 4.09 to 3.73.
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By NoShortcuts - 6 Years Ago
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Ford used 3.70:1 rear end gearing behind the '57 'Bird D code 312s running the T-85 w/ O/D transmission.
Consider the cam you're running, the weight of your wagon, your tire diameter, and how much low speed stop and go driving you do.
Hope this helps. 
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By miker - 6 Years Ago
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I’m not sure about an open differential, but on a posi unit 3.73 is the breaking point. i.e. if I went from the 3.73 in my bird to a 4.09 or whatever, the posi unit needs to be changed also. And my bird does have a 44,probably out of a truck somewhere in the past.
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By 62bigwindow - 6 Years Ago
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Yes, I'll need a new carrier. The 44 came in T Birds too and from what I can tell so far it's the same as the 44 that came in wagons. As far as specs my tire diameter is 28.5, cam is 236@50, and I mostly drive on the highways. Some town driving but not much.
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By PF Arcand - 6 Years Ago
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It's your call, but that's going to be a big bill to change the entire rear axle assembly.. You've got O.D. already, way cheaper on fuel & easier on the engine to just reduce your cruising speed 5-10 mph..
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By 62bigwindow - 6 Years Ago
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Unfortunately it's going to have to be opened up anyway. The pinion bearing is starting to make noise and the gears have alot of slop in them. If it was in good shape I'd probably do exactly as Paul suggested
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By 2721955meteor - 6 Years Ago
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have a new in the box 3.73 gears dana 44. 150 us. ct1940@shaw.ca
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By Ted - 6 Years Ago
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62bigwindow (4/20/2018)
The rear axle in my car is going to need a rebuild. With a T85 OD what would be a good ratio to swap in? I'd like to lower the cruising rpm at 70-75 but not sacrifice too much low end. I was thinking maybe switching from the current 4.09 to 3.73.
My ’55 Customline originally came with 3.89:1 rear gearing with the original overdrive tranny. I’ve since installed the T-85 Overdrive (same gearing as the original tranny) and replaced the rear end gearing with a set of 3.56:1 cogs. The 272 is really happy with the ‘more highway’ orientated or ‘taller’ gearing.
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By 62bigwindow - 6 Years Ago
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Thanks Ted. I was hoping to get some feedback from someone who had made the swap. 55 meteor I'll be in touch soon.
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By NoShortcuts - 6 Years Ago
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62bigwindow. I'm not sure what camshaft, cubic inch displacement, or compression ratio factors you've got with your engine. You indicated that the camshaft duration is 236 degrees @ .050.
John Mummert has a Y-284-S cam listing that shows 236 degrees @ .050. That Mummert listing also shows an advertised duration of 284 degrees, a 108 degree lobe separation angle, and a 68 degree intake and exhaust valve overlap. These are numbers I equate with top end performance not street-ability drive-ability.
IF JM's Y-284-S cam listing is akin to what you have in your engine, I suspect that the low end engine torque is marginal for stop-and-go level terrain driving and may be exasperated by start-up on any significant incline. My suspicion is that your 4.09:1 rear gears make low speed performance of your '56 wagon workable and support low speed drive-ability.
Other variables may come in to play with what you're experiencing in the drive-ability of your '56 wagon. My hobbyist experience tells me that a given aftermarket performance camshaft will exhibit very different drive-ability performance in a 272 cubic inch y-block than it does in a .060 over-bored 312 (322). Similarly, the effective operation of a performance camshaft is tied to an engine's static compression ratio and the dynamic compression ratio the engine actually operates with on the street.
Accomplishing street-ability with a high performance engine while dropping engine speed for highway use is a challenge. Your T-85 R-11 overdrive offers a .72:1 final drive ratio with a 2.49:1 first gear. Consider looking at a TREMEC Super Duty T-5 part no. MD 552-0251 from Modern DriveLine. 5th gear final drive ratio is .63:1 with a 2.95:1 first gear. I think this transmission would work better for your application with either a 3.73:1 or 3.56:1 rear end because it has a lower 1st gear ratio and a higher final drive ratio than the T-85 offers.
Hope this helps!
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By 62bigwindow - 6 Years Ago
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Your dead on about the cam. It does pose some low speed nuances. Especially with 292 ci and only 9:1 compression. Id love to yank it out and swap in a more street friendly profile but we all know how easy that is with it in the car. I have actually thought about a T5 but it's not in the budget. Also a three on the tree is a good theft deterrent. Thanks for all your help and advise.
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