Question-------------------------
Followup To
Question -
Dear Dick,
I have a 1952 Chevrolet pickup that has had a 235 cubic inch six cylinder put in it. Currently it has a three speed manual transmission but I am considering hooking up the old GM two speed automatic to it. Do you know what gear ratio I should have in the differential? A smooth, economical ride is my objective. Thanks in advance.
Bill Barker
Answer -
Your truck has the original driveline, probably, with the enclosed driveshaft. Being a truck (I'm going to assume it is a 1/2 ton), it probably came with something like 4.11 gears (I don't have info on trucks) and this makes it a 45-50 MPH truck, especially with the original 216 splash oiler engine. With the 235, you might be able to drive it 60 for a short spurt while now, but that's really spinning that old mill.
If you are going to put a Powerglide in it, the engine will turn even faster than it does now, due to the slippage in the torque convertor, even in high gear. Therefore, you need to look for a much lower gear ratio. Since you are going to be putting a different driveline in it, you are most likely going to wind up with a passenger car rear end - so I'd suggest something in the 3.54 range, considering the capability of the engine. This might be a decent road vehicle, but it isn't going to be much of a heavy hauler anymore, and it's going to be bog slow off the line with that powerglide!
I drove a '51 1/2 ton standard cab with the same setup you have (235/3 on the tree)for over 20 years from 1975 to 1997 - it was a great truck, and didn't complain about some really outrageous overloads I hit it with from time to time. Of course, it was a 45 MPH truck - above that it was really straining things.
Good luck with it, I really still miss mine (love that whine in second gear!)
Dick
Dear Dick,
Thanks for your timely and informative response. I suppose I should have included more info with my original question. You are correct in that the original truck ( a 1/2 ton ) had 411 gears, a splash rod 216 and a closed driveshaft system. Today it has a 235 engine, an open driveshaft, 3 on the tree, and an unknown (to me) rear end gear ratio. It has decent acceleration, a whine in second gear, haha, and at about 60 mph, all the rpm that I care to subject it to. In the early '60's I had experience with the 283 V8 and Powerglide combination in a number of cars. Dependable, good top end, but a dog off the line. I have a 1948 Chevy 1/2 ton that came to me with a 250 six cylinder (a tight fit) and a three speed automatic, rear end ratio unknown. This is a good combination but in order to hook up a three speed trans to the 235 in my '52 I'm told that I will need a special converter. I'm exploring what to do here. Put in the three speed w/converter or go back to the old Powerglide...and what rear end ratio to go with. Do you know what rear end ratio the early 60's passenger car had? Someone suggested a rear end ratio of 338. Too much, right? This truck will be a daily driver for my wife and will not be subject to much work. I'd like it to have ok acceleration and be comfortable around 60-65 mph. Thanks in advance for all your help.
Bill Barker
AnswerI don't have much to add here, just an observation or two, which are probably not news to you anyway:
The 235 was only mated to the Powerglide or the 3 speed, so there won't be any interchange to a more modern automatic, except maybe some aftermarket adaptor, if you could still find one (they're probably all retired by now). Most folks who did this sort of thing were changing the 6 to a V8, not putting a different transmission on a 235.
Regardless of what transmission you put in, if it is older than one of the modern ones (like a 700R4 or a 4L60) with an overdrive, the RPM of the engine is going to be exactly what it is now - because they all approach 1:1 when up to speed. With that in mind, I don't think your truck will be able to pull anything lower than a 3.54 - which might be what is in there now, from what you say. I think there was a 3.31, but I don't think the 235 would be able to pull it worth a darn. Why don't you measure the ratio (jack up one wheel and count the turns of the wheel for one turn of the driveshaft, then divide by two (because the other wheel isn't turning).
If it is higher than 3.54 see if you can figure out what car the rear end is out of, and maybe you can hunt for a different pumpkin for such a car with a higher speed rear end.
It's beside the point for this discussion, but I have a tow truck I use here on the ranch, it's a 52 Ford(!) 3/4 ton, with a Chevy 348/Powerglide with a 5.36 rear end. This thing will flat stand up on its hind legs if I get on it. I've even moved buses with this thing! I never take it out on the road, but I think it would run out of breath at about 30 MPH. I also have a utility 4X4 truck, an ex-Army 65 Ford, with the little 240/6, 4 WD HI/LO range, and 4:11 gears. On the very rare occasions when I take it out on the highway, I am scared to death, because our highway is a 2 lane/70 MPH road, and this thing sounds like it is about to throw a rod at anything over 55. I have to drive on the right shoulder with one eye in the mirror. I'm sure the Army guys just let it roar, but I just can't bring myself to do that. I guess it would go 65 for a little while, but then I'd have to go back and pick up pieces!
My 99 GMC has the 4L60 and 3.70 gears - it is turning only 1500 RPM at 53MPH - or 1700 at 60 - it's really nice!
Your '48 with the 250 sounds like a good combination - a much more modern engine, willing to spin a little faster without strain. They also had the 292, a great engine, one that could really do some work!
Good luck with your projects!
Dick
