Okay, so it's definitely a '76 Overdrive transmission. Next question is on tire sizes for use with this transmission. I have a complete set of stock 14" wheels. What tire size and rear end gears would be best for a car I can drive at interstate speeds without revving the engine too much, but still be able to perform well off the line? I'm also interested in posi-trac, which I do not currently have. If anyone has this item laying around I'd be interested, either a complete rear end or just the differential.
With a .73 overdrive you can run pretty much any rear gear you want to. But there is such a thing as too much starter gear for the street .
The key with that overdrive box is to get second gear right for your application and driving style.... cuz you are gonna be spending a lotta lotta time in that gear, on account of third is so far away, in terms of rpm.
So that takes me back to post #12.
Oh yeah, tire size; I highly recommend the tallest tire you can physically fit in the wheelwell. And if you like flipping around corners, then also the widest. My 68 Barracuda has the same wheelwells your Duster has, and while it is physically possible to install 325/50-15s, I don't recommend it for the street. Next best idea is 295/50-15s, but you will have to move the springs inboard a lil and probably requires custom offset wheels; unless you narrow the rear.
A direct fit is 275/60-15s for straightline work, and 275/50-15s for a modicum of stability around city streets. But be advised that neither of these in BFG/Coopers/etc are particularly able to handle more than 300/maybe less horsepower, on the street.And if you get the gearing right, they are like roller skates, so take it easy in the turns until you get a handle on them.
There are stickier compounds.
With a 340/4-speed, forget 14s. They will put you in the weeds every time you open the secondaries in anything but a straight line. And that gets old real quick.
4.10 will cruise at ~2400, but
4.10s x3.09= 12.67 starter gear, which normally is a PITA . but
4.10x 1.67 second gear is 6.85 roadgear and will get you
~2930@35,and 3770@45,and 4600@55,and 5440@65 So you probably can't run much more or less gear with 27/28 inch tires. 4.30s tops
I ran those, but first gear is crazy; in normal driving, you have to shift into second almost right away; and then there you are at some ignorant 2190@25, or 1300@15 mph.
So what I'm saying is to make that trans work with a 340, make sure she has some stones down low in the rpm band. If you put a 268/110 cam in there at 9/1 Scr you will be extremely unhappy until the power comes on at something like 55mph in second gear. First don't count on the street with a 12.67 starter gear, she'll just annihilate the tires.Then when you shift, the Rs will fall to 54%. If you outshift first at 5500, the Rs will fall to 2970@35 mph (in second), easily 1000 to 1200 rpm below the torque peak, so then you wait....... and wait ....... and wait.
Well there is one way but I wouldn't do it; run first gear to whatever rpm you dare. Say with 3.23s.
The starter is now 9.98 very nice with small tires. And with 26.5 tires first will get you 50mph @6330. And 65 in overdrive will be 1950. So on paper that looks good. But 30 mph is 3800 and you won't be cruising there, lol. In second, that same 30 is now 2050........ so if you need to accelerate, that ain't gonna happen real quick.
And when merging onto the freeway at say 45 mph in second, the Rs are now 3080, so you will want to downshift....... but that will get you 5700 in first gear! Umm no, downshifting is not an option. So there you sit with the gas pedal on the floor at 3080, again waaaaaay below the torque peak. So I don't recommend 3.23s .. Been there done that, not going back.
Well I suppose, you could run a 318-type cam, and get the torque peak down to something like 2800,lol.
Honestly; I highly recommend not to use this trans with a 340, the splits are just too wide. I didn't even like it with my HO 360 which had gobs and gobs of low-rpm torque. I tried it with every rear gear from 2.94 to 5.13, and the only way I could remotely like it was by putting a GVOD behind it and splitting gears with 4.30s in the back. Yeah that was a hoot.
A 340 likes the TA box; 2.47-1.77-1.34-1.00 and with a 3.91,(or maybe a 4.10). Now you've got a tiger by the tail. But cruising at 65=3100 kindof sucks. Worse with 14s.
