The Stovebolt Site Forums The Shop Area Driveline Rear end gearing

Hi Folks. I need some suggestions from those more learned than me on rear end gearing. My 51 3100 has a .60 over 235 with a Patrick's 3.55 setup in the torque tube. At 50-52 mph she says, "I'm good right here". Can't hit the freeway with my fellow rodders. I've decided to put a tremec 5 speed (American Powertrain) in place of the SM420. I picked up a 1978 Camaro 10 bolt 8.5 for a rear end. Looks like my choices are 3.42 or 3.73 for a ring and pinion. Any edumacated opinions on which to go with?
P.S. Has anybody had experience with the front disk setup from CPP Products?

Thanks for any help,
Bodini

Given your tires are 26 in tall with your set up 50 mph you'll be turning 2,500 rpm , just coming on its power band , 3000 rpm will give you roughly 65 mph
which will make the 6 cyl music a little louder , but shouldn't bother the motor if its in fair shape .
Sounds to me that you might over gear your truck to the point that you'll loose performance.
What's the over drive ratio on your Tremec

John

3 or 4 speed? I have a 1950 3100 torque tube, with 235 & original 4:11 rear gears, 3- Speed and she goes 60-62 (per GPS) before starting to make lots of noise. These engines were meant to rev up! I do hit the freeway but only for short distances.
Todd

Edit sorry just read SM420. 4 speed

It's a Tremec TKO 500. The 5th gear OD is .68

Just for grins, take the fan off and make a highway run. I believe you'll figure out pretty quickly where the noise is coming from. Unless you're running ridiculously low profile tires, the 3.55 rear end you've got already is just about ideal. There's nothing whatsoever wrong with letting a 235, or even a 216 wind up where it's designed to run, as long as the bottom end of the engine and the oil pump is in good condition.
Jerry

Not going to be in 5th very much with that kind of ratio! No hills in Arkansas?

My experience is that 2200~2300 rpm is the perfect cruising rpm for the 235 in an AD truck. On flat road without wind, you can drop that to 2100 or maybe a bit less. With a 3.55 and 27 inch tires and driving 50~52 mph, that is exactly where you are...2200 rpm. And that's why your engine is saying "I'm good right here." With this in mind, let's work backwards from there and see what you have. That .68 fifth gear will be challenging...no questions about that, but with the 3.73 rear and 26 inch tires, you'll be spinning a little over 2100 rpm at 65. That is probably workable, but you need to understand in wind and hills you'll be downshifting to 4th. And...another positive...your starting line ratio if you have the 3.27 first gear (which was common in transmissions with the .68 fifth gear) will be 12. Which for a 1/2 ton pickup is perfect. Good luck.

Best site I’ve seen to
Calculate speed vs rpm for different gear ratios and tire sizes is https://4lo.com/calc/all-calcs.php

Using a calculator eliminates all of the guesswork and opinions. Use the calculator to find the exact rpm you say it’s happy with right here...
And plug in different trans and axle
Ratios and tire sizes to get to that sweet spot at 60, or 65 or whatever speed your hot rodding buddies run at...

When I ordered my TKO 600 I got these gears. I like the higher 1st, (vs the 3.27) and the higher OD, (vs the .68). I don't know if I'm saying that right but you get the point. I'm running the torque monster 292.
1st-2.87
2nd-1.89
3rd-1.28
4th-1.0
5th-.82

Great idea. Nothing wrong with running without a fan as long as prolonged idle is avoided. Especially in the current winter months on a vehicle that's just going down the road and not working. Once up to cruising speed the fan is just along for the ride anyway. I've been down the 1/4 mile many times with the fan in the back seat when a few extra hp were required. Much more recently my Dodge Diesel (with it's sized to do serious work radiator) used to regularly see the fan pulled in November and reinstalled in March. Two reasons, one it got much quitter and two the engine heated up quicker on those Cold days.

I'd definitely say pull the fan and take a drive, just watch the temp gauge.

One last thought, since doing a lot of work insulating the cab along with adjusting the valves to the tighter end of the adjustment range my '51 with original 216 is much nicer on the road at speed. At speed means 60-65 mph. Problem at speed is just not enough motor to stay there with any headwind or uphill grade (no real mountains in Arkansas but lots of big hills in the NW corner) it's far from flat unless you're in the Delta.

Good Luck,

RonR

Kinda sounds like I can just leave her in 4th driving around town and then drop her down into OD if I want to get on the freeway.

Ron, if you just haul a couple of razorback hawgs around in the bed, their squealing will drown out any engine noises!
LOL!
Jerry

How are you going to attach a Tremec to a 235?

Another data point :

215/65-16 from 215/85-16 will almost mathematically negate the 4.10 to 3.55 ........as far rpm on the highway cruise.

And a good point it is!
Most tire manufacturers show rev per mile of each tire on there website, makes it easy to do the math,.

American Powertrain and Modern Driveline both have a kit to bolt up to the 235. Tires on the back are currently 235x65x15, about 27" tall on original 15" wheels. I'm thing of getting 235x60x16 tires so I can switch over to a bigger 16x6 mag type wheel.

The band-aid tires are your problem. Get some 16" wheels and tires with real sidewalls and increase the rolling diameter, and your problems will go away. The Ram 1500 I use on business had the opposite problem- - - -the 17" wheels and tires that were on it when I bought it made the cruise speed "sweet spot" about 75 MPH. When I tried to cruse at legal speeds on the interstate, the transmission would constantly toggle in and out of torque converter lockup, and it would occasionally drop out of overdrive into direct. The solution was to install 16" wheels with 70-profile tires to drop the road speed at 2K RPM where the engine likes to run. It resulted in a 5 MPH error in the speedometer according to my GPS, and nearly a 2 MPG improvement in gas mileage because the engine quit lugging down and opening up the full power calibration of the computer controlling the injectors.

Your situation is different- - - - -you need a taller tire and bigger wheels to give you more road speed for a particular RPM.
Jerry

Concur on the big tire solution. I recently put 16" Dexstar 8 lug wheels on my 1949 3600 (261 and SM420), and shod those wheels with 235/85 R16 radials. That gave me a tire diameter of 31.7", which sounds enormous, but in fact just matches the diameter of the optional 17" wheels offered in 1949 (31.5" from the factory sheet). Tires were only $100 each, but I couldn't find anything less than 10 ply (in the 85 ratio). 50mph generates about 2500 RPM and 2900 RPM yields about 60mph. A big change from the 235/75 R15's that were on before.