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Old 11-02-2002, 06:09 PM
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Post ideal rear end gears for pulling

I have a 1972 Chevy Truck that does alot of hauling and pulling heavy trailers. I have a 1980 350 SBC in it with around 250-275 HP and close to 350 TQ or better. It has a TH-350 transmission with stock torque converter. My question is, what would be the best rear end gears to install??

It has stock 3.08 now (not the best for pulling stuff). I would like one that pulls really good and is fair on the highway. I was thinking of 3.73, but not sure, and maybe an aftermarket torque converter. Any help would greatly be appreciated! Thanks!

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Old 11-02-2002, 07:11 PM
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The real question is where do you want your engines rpms at on the highway. 3.73 don't tell me anything without knowing variables for tire sizes. But if I had to take a guess if your running 3.08's now whatever rpm your engine is at now going down the highway add about 1,000 rpm or so to that with the 3.73's. If you think that's too high for you being concerned about highway driving with no overdrive trans maybe 3.55's would be better. You need your tires diameter to actually calculate what your rpm's will be with a calculator program, or you can guesstimate.
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Old 11-02-2002, 07:56 PM
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All I know is, that they are 235-75-15 tires on the back. Right now, it is running about 2400 rpm at 60 mph. I don't do that much highway driving, except local highways, not going above 60-65 that much at all. I want something that gives alot of pull for hauling trailers and such!

[ November 03, 2002: Message edited by: stfinney ]</p>
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Old 11-03-2002, 09:37 PM
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...lets see, towing power, under 60mph...
.......I'd say 5.13s and a 4000rpm stall converter.

......yeah, that'd be adequate

no but really, i'd say 3.55s and a stock converter. I think a looser converter would just slip alot when towing a trailer causing a lot of heat. I would get a tranny cooler too if you are doing a bunch of towing.
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Old 11-04-2002, 03:55 PM
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Thanks! I already have a trans cooler built in to my radiator. It came standard on this truck. Most of my driving is in the idle-2500 range. So, what would be a good stall speed for the converter? I am going to get a better one, instead of the stock converter.
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Old 11-04-2002, 04:27 PM
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If your towing you want low end torque. This means your engine should have a mild to moderate cam and intake matched to it. With this set-up there's no need for a high stall converter. Towing builds up heat in your trans fluid, high stall converters build heat in trans fluid because of added slippage, put those two together and that's a lot of heat. But that's just my opinion. A high stall converter should only be used because your engine idles at 1,200rpms and needs it. And if your engine's that hairy it's not really set up for towing. I would go with the lowest stall speed possible that still allows your engine to idle in drive without stalling or loading up.
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Old 11-05-2002, 03:21 PM
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I swapped out Dads rear gears for 4.10:1s,
he doesn't go fast, but hauls heavy loads.
The original gears were 3.08s, so the off the
line was a huge improvement, and the tire size
was 30/9.50R15s...

Steve & the Rockette...
Too many to list...
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Old 11-05-2002, 06:16 PM
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Thanks for the info! I am torn between 3.73 and 4.10's......LOL

It is a fairly heavy truck anyway. They built them good and strong back then (1972).

[ November 05, 2002: Message edited by: stfinney ]</p>
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Old 11-06-2002, 06:08 PM
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Yeah man,

I'd say 3.73's or 4.11's with a full lock-up converter and you be well on your way to hauling tanks.

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Old 11-12-2002, 07:30 PM
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You still need more trans cooler! The one in the radiator is adequate for normal driving and light towing only. Get an aftermarket type that mounts in front of the radiator. Cut the return line from the built in cooler and run it into the inlet of the aftermarket cooler, then to the trans.

I'd stick with no more that 3.55 gears without an overdrive transmission. That will still raise engine rpms around 500, but that's not to much on the road. 3.55 should pull a decent load without straining the engine at all. The Jeep Cherokees (the little XJ) uses a 3.55 rear axle and has a 5,000 # towing capacity with the 190 hp 4.0L in-line six.
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Old 11-15-2002, 02:49 PM
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Stfinney: I have written a little Excel Code to calculate changes in tire size and axle ratio. Here is what I get:

For a 235/75/15 tire and 3.73 gearing:
first no. is rpm, second is mph
3500/82
3300/75
3000/70
2800/65
2600/61
2400/56
2200/51

For the same tires and 3.55 gearing the results are:
3500/86
3300/79
3000/73
2800/69
2600/64
2400/59
2200/54

If anyone wants the program and is able to receive and use MS Excel, I will send it to you upon request. You will have to provide your e-mail address.

I have tested this program against my own gearing and tire size and it holds up in a measured mile within 1mph assuming that I am reasonably able to hold a constant rpm and write with one hand.
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Old 11-15-2002, 02:58 PM
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My input on this is that you want to avoid low rpms for towing because you simply lose the benefit of port velocity as the truck bogs down on a hill and you end up down shifting and putting the pedal to the floor which eats gas like crazy. You are better off maintaining a higher rpm that can be sustained in most driving conditions. You won't end up constantly increasing your speed to compensate for bog down.

Even if a higher rear end turns the motor faster, it take less throttle opening to perform the work, so you aren't spending that much more on fuel, only the component that is necessary to power the additional valve openings and other internal engine frictions. So fuel should not increase greatly and in fact if you are geared with a numerically low gear now, you may find that your fuel consumption actually decreases because the engine atomizes the fuel better.

Now if you wanted to pull the tounge off your trailer at the stop light (you know the Dodge Ram Hemi commercial which drag races young guys while pulling the hemi on the trailer)you would build that Chevy 350 as a 383. You would have over 400 ft lbs at stall speed. Mine is wickedly powerful.
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