rear end ratio - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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  • 1 Post By Malibu ss 64
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post #1 of 12 (permalink) Old Jun 26th, 19, 7:51 PM Thread Starter
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rear end ratio

I have a 72 El Camino with a 454 (500 hp with a decent cam) and a Th400 built by ATI. I have a 2800 stall and 12 bolt posi 3.73 gears in the rear end. I have MT 28/12 x 15 rear tires. I currently am hitting 3,000 rpms at 65 mph.

I am considering changing the rear end gears to a more user friendly ratio of either 3.31 or 3.42. The car is never raced.

I am hoping to drop the rpm range to a more comfortable range of 2500 to 2700 range. Also hopefully help my MPG.

Any thoughts???
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post #2 of 12 (permalink) Old Jun 26th, 19, 8:06 PM
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I have a 3.31 in my 12 bolt with with stock wide ovals that are not quite 27 tall. I cruise at 70 mph at 3000 rpm. That is until I’m done with my 5 speed install. You should get a little more speed at that rpm with your taller tires. But to be honest, with my 402 big block with 420hp and 500tq your mileage may not improve much since I currently get about 13 without my foot in it most of the time. Drone will be better though.
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post #3 of 12 (permalink) Old Jun 26th, 19, 9:04 PM
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Re: rear end ratio

Not enough ratio change to notice the difference. My 2 cents.
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post #4 of 12 (permalink) Old Jun 26th, 19, 10:55 PM
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Re: rear end ratio

I have 3:55's with 28" tall tires and run 2600 rpm's at 60.

71 Malibu, Cottonwood Green, 396 BBC, AutoGear M22W 4 speed, 12 bolt 3:55 posi.
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post #5 of 12 (permalink) Old Jun 26th, 19, 11:42 PM
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Re: rear end ratio

Quote:
Originally Posted by The Bullet View Post
I have a 72 El Camino with a 454 (500 hp with a decent cam) and a Th400 built by ATI. I have a 2800 stall and 12 bolt posi 3.73 gears in the rear end. I have MT 28/12 x 15 rear tires. I currently am hitting 3,000 rpms at 65 mph.

I am considering changing the rear end gears to a more user friendly ratio of either 3.31 or 3.42. The car is never raced.

I am hoping to drop the rpm range to a more comfortable range of 2500 to 2700 range. Also hopefully help my MPG.

Any thoughts???
3.31 axle will have you turning 2582rpm at 65mph, with no converter slip...
using this link: Wallace Racing - Calculate RPM for given MPH, Rear Gear Ratio and Trans Gear Ratio

68-Malibu 427bbc/t4oo/3.73-12bolt/pro charger f1-r
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post #6 of 12 (permalink) Old Jun 27th, 19, 4:17 AM
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Re: rear end ratio

I went from 3.73 to 3.31 and is was enough difference for me to make it more liveable on the highway.
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Chevelle Malibu ss 64
489 BB with parts from Lewis racing
Alumitech radiator with dual spals
Th 400 Edge 9.5" converter
57 3/4” wide 12 bolt: Eaton posi, Us gear 3.31, Toms KA 33 splines axles
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post #7 of 12 (permalink) Old Jun 27th, 19, 7:59 AM
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Re: rear end ratio

Quote:
Originally Posted by Malibu ss 64 View Post
I went from 3.73 to 3.31 and is was enough difference for me to make it more liveable on the highway.
go to 3.31 if you want a noticeable difference. 3.42 won't be.
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post #8 of 12 (permalink) Old Jun 27th, 19, 8:58 AM
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Re: rear end ratio

If you never race maybe a 3.08 is the ticket? You have enough engine to handle it.
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post #9 of 12 (permalink) Old Jun 27th, 19, 9:47 AM
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Re: rear end ratio

Quote:
Originally Posted by The Bullet View Post
I have a 72 El Camino with a 454 (500 hp with a decent cam) and a Th400 built by ATI. I have a 2800 stall and 12 bolt posi 3.73 gears in the rear end. I have MT 28/12 x 15 rear tires. I currently am hitting 3,000 rpms at 65 mph.

I am considering changing the rear end gears to a more user friendly ratio of either 3.31 or 3.42. The car is never raced.

I am hoping to drop the rpm range to a more comfortable range of 2500 to 2700 range. Also hopefully help my MPG.

Any thoughts???
If you are just driving it around town and not racing it, then I would honestly consider 3.08's for the gas mileage.
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post #10 of 12 (permalink) Old Jun 27th, 19, 2:08 PM
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Re: rear end ratio

I'm going with 3.08s, this Doug Nash has a deep enough first gear to still give good off the line acceleration.

Later, dozer
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post #11 of 12 (permalink) Old Jun 28th, 19, 12:35 PM
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Re: rear end ratio

I went from 3.73 to 3.08. Made the car much more enjoyable to drive even with a 2.20 low M22. There was a performance trade off however, but in my case it was worth it.

Jon

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post #12 of 12 (permalink) Old Jul 1st, 19, 8:21 AM
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Re: rear end ratio

I have 3.08's in daily driver and get good gas mileage and the acceleration is not bad as well.
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