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Transmission and DriveLine Questions
Ask all your Transmission and DriveLine Questions. Tranmission, Drives shaft, Rear End, Gears, etc
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Old 07-17-2017, 04:47 PM   #1
DavidY
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Default Rear end for a stock six

I have a six cylinder, three speed manual with the stock 3:55 rear end. The engine is very slightly modded with 246 cu.in., dual rochester one barrel carbs and a Fenton dual exhaust so I'm getting slightly more power than stock. Around town I have plenty of power - I can accelerate up hills in high gear and can make effortless second gear starts.

My problem is that i feel like I should have another gear when I've got up to normal speeds. It's almost like I have too much power and it wants another gear to shift into. So, my question is, would a 3:36 be better, or would it be not enough of a difference to matter? How about a 3:08?

I could add that my previous V8 had a Powerglide that I destroyed and replaced with a three speed but not the rear end. That car ran happily with the 3:36 rear but of course the V8 had a bit more power than my six.

I'm NOT going to replace the engine with V8 and I'm NOT going to replace the three speed with anything else so those suggestions are off the board. I'm only asking about the rear end gears.
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Old 07-17-2017, 04:54 PM   #2
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I think you answered your own question, a 3.55 is too small change which leaves 3.08.
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Old 07-17-2017, 05:56 PM   #3
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If the hills aren't big in PA. 3:08 should be fine. If they held a really big swap meet near you the transmission to find would be a overdrive 3spd. Same linkage,no holes cut in the floor.Easy swap for a guy who doesn't want to swap anything.
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Old 07-17-2017, 06:40 PM   #4
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So, your first gear is 2:94 or something close and your rear gear is 3:55 so you have a 10.43 effective first gear which is what gets a low powered 3500 pound car off and running from a stop. The final gear is 1:1 so 3:55 for cruising. You have to be careful with lowering the gears to say a 3.08 in that you may very well end up with a slug that either lugs the motor trying to get going or you have to slip the clutch until you get up and going so as not to lug the motor, neither are good for engine or clutch. Lugging the 6 is never a good idea because of the length of the crank and the stress lugging it puts on it. It is a cast steel crank, but its old and can sometimes be not so forgiving. You will lower your rpms at highway speeds but lose the take off from a dead stop. A 3:36 gear lowers your effective first gear to 9.87 which is not a terrible loss but in the end, you will still feel a little difference getting yourself going from a stop, as well as your downshifting, and you loose effective gearing in 2 but will have a modest drop in rpm at highway speeds. I am not so sure that you will see that much of a difference though.

Its a compromise no doubt. I drove my 57 (I have owned it for 40 years) with a stock three speed and a 3:55 rear for many years, put probably 50,000 miles on the car in that configuration in the 70s and 80s and that 6 motor just kept going and going, as they do.
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Old 07-18-2017, 01:21 PM   #5
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Im going to do a little guesstimation, but for the purposes of a comparison; the end result should make sense.
using this calculator for cruising RPM: http://wallaceracing.com/calc-gear-tire-rpm-mph.php

a 3.55 rear, with a 26" tall tire and assuming 60mph...mathematically nets you 2752rpm

a 3.36 rear, with a 26" tire and 60mph...nets you 2605rpm

a 3.55 rear, with a *27" tall tire* and 60mph... nets you 2650rpm.

So; save your pennies for the 3spd/OD and buy taller tires in the interim maybe?

I would NOT go 3.08s with the 2.94 1st (thanks for that tidbit DragSix) and an I6. Maybe if you could find a 3spd with a 3.2x 1st; but that makes for a huge compromise in sporty driving.
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Old 07-18-2017, 03:18 PM   #6
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Don't know how hard it is to come up with an overdrive but that looks like the best of all worlds.
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