The Stovebolt Site Forums The Shop Area Driveline Trans I should use with a 3.54 posi open rear

I have a rear out of a 60's suburban that has a 3.54 gear in it. I have a 61 235 that it will be behind. Just wanted to ask you guys what you think.
I also have a MY6 trans and a center section from a 59 1/2 ton with 3.90 gears. Would that be a better way to go than putting the sub's rear in?
Thanks,
Tim

The MY6 is a New Process overdrive. I think it would do better with a 3.90. 3.54 is too fast with a 235 with overdrive. What kind of transmission do you have now?

I agree with crenwelge. I'd run an RPM calculator and see your rpm is at cruising. I suspect you'd be around 22-25 hundred rpm, maybe a little high, but not enough to warrant an OD. You might loose mileage with an OD.

Larry

I just have the stock 3 speed and rear end.
So it would probably be better to use the MY6 with the 3.90 pumpkin from the 59.
I know the rear from the suburban has too much gear for the OD trans, just wondering what trans would go with that sub rear?

The 3 speed that your vehicle was born with would do fine in front of a 3.54 rear. And a lot less expense, and the MY6 would require rigging some linkage.

Tim,

There are a couple things you need to consider about opening up the drivetrain and gearing. You need to tailor you gear selection to the type of driving/cruising you personally will do. I'd suggest you use this calculator to crunch your potential transmissions final drive numbers, rear tire diameter, and ring and pinion ratio. I like this calculator:

http://csgnetwork.com/multirpmcalc.html

Larry has pretty much pegged the 22-2500 rpm range as the most efficient rpm for cruising. That's where you want to do most of your cruising and give it plenty of thought. I set my R&P ratio/cruising speed/tranny final at 70 mph @2000 rpm's and discovered that my vacuum doesn't max out until I'm running almost 80 mph. In addition, my buddies cruise at 55-60 mph so I'm running at the bottom of the torque curve (1500-1600) when I'm putzing along with them. The engine will pull in 5th gear at that low rpm but it's very inefficient for fuel mileage. If you have your cruise speed@ 22-2500 rpm you'll still have 300 rpm to get you up into the 70 mph range which with a decent running stock 235 shouldn't create any issues.

Something else to consider with adding a differential is the width and getting rims that will fit properly into the rear wheel wells. I've used the Camaro 88 and 89 diffs (62" wide...mounting surface to mounting surface) in my 54 truck and have had to purchase aftermarket rims to get the correct "look" and lug pattern. No big problem if that's what you budgeted for.

As important as getting your gearing right you need to do your homework and leave the "opinions" to other folks. Figure out what cruising speed you'll want and match your gearing, both transmission and rear end to that end.

You might check out my webshots albums for some of the how-to's on changing to an open driveline. Hope this helps with your plan.

Dave

Dave, good post. The only thing that hasn't been mentioned is individual gear ratios, 1st, 2nd, 3rd, and 4th. The final drive whether it is 3rd, or 4th will be 1:1, unless it's an OD. I don't have enough experience to make a comment as to which trans. If you keep the ratios fairly close to your 3spd and no big jumps between any gear you should be OK.

Larry

Are you inquiring about a 51 model or what? If it is a 51 model, you will need an open transmission from a 55 or later. Probably the simplest switch is to put 3.55 gears in your current third member. Have you measured the tread width of the suburban rear? It may have 5 hole wheels. Some people don't mind having mismatched wheels.

I have a 64GMC 3/4 ton that had 4.11 gears in it & at 70mph I was turning 32-3300 rpm. I had 3.53's installed & it dropped to about 27-2800 rpm's.I have a 350 in it so that may make a difference I don't know. Isn't a my6 a 4speed with 4Th as a overdrive & 3rd a 1:1, just use 3rd unless your on the hwy at sufficient speed.

Yes I have a MY6 or A833 with OD in it. I can throw that in the truck with the 3.90 59 pumpkin that I have. Should work good.
I also have a 60's suburban rear with posi 3.54 gears and 6 lug. It's perfect width for the 47 to 54 1/2 ton's.
I just wanted to know what trans. would go with that rear. It can not use the original 3 speed because it is an open rear.
I think I will sell that rear and also an SM 420 trans. I have sitting here too.
Thanks everyone.
Tim