Rear Gear Recommendation - Chevelle Tech
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post #1 of 28 (permalink) Old Jan 21st, 20, 6:41 PM Thread Starter
 
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Rear Gear Recommendation

Need recommendation on rear gear ratio for my 1964 Chevelle - 383 stroker motor @450HP , TH350 trans, 12 bolt rear. Car came with 4.26 rear gear.
Looking to change to a gear that will give be better cruising, but still with some bite. Would a 3.70 rear gear be a good option?
Thanks.
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post #2 of 28 (permalink) Old Jan 21st, 20, 8:18 PM
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Re: Rear Gear Recommendation

what tire height are you running? What speed would you like to cruise at and for how long?
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post #3 of 28 (permalink) Old Jan 21st, 20, 8:23 PM
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No.
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post #4 of 28 (permalink) Old Jan 21st, 20, 9:18 PM
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Re: Rear Gear Recommendation

336 is a given to make the numbers work out.
336 x gear ratio x MPH divided by Tire diameter = RPM.
Now add 10% for converter slippage and you will be really close.

I have 370's in my 3600lb 57 chevy with 350" and TH350 it works very well. but as you know many highways have the fast lane going 80 MPH and that 370 gear will have you at 3900-4000 rpm in high gear.

I love my 370 gear as it pulls hard and does excellent with boat in tow.
I stuck 3.00 gears in and it was a bit of a pig with the 20 foot pontoon behind me ..Found myself reaching for second gear a lot.
I have another V8 vehicle with TH 350 and 3.25 gears and it is really nice cruising in .. but it weighs 3176 lbs.

Father-in-law had a 64 and we got a different complete rearend one for cruising gear and one for a race gear.

Took 45 minutes to swap the complete rearends.
You may go that path if you have a place to do that stuff.

Now currently at 80 with 28" tire and your 4.26 gear you would be about 500 rpm higher than the 3.70
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post #5 of 28 (permalink) Old Jan 21st, 20, 9:19 PM
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Use this calculator, it will help you see your complete scenario for cruising, shifting, etc. There's a lot of ads so pay attention to where you click.

https://www.blocklayer.com/rpm-gear.aspx

*Jeff*

Project Salty - 1964 4 door Malibu, beaten, neglected, red headed foster child
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post #6 of 28 (permalink) Old Jan 21st, 20, 9:28 PM
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Re: Rear Gear Recommendation

You probably want at least a 20% change. I’d go 3.31 to 3.42.
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post #7 of 28 (permalink) Old Jan 21st, 20, 10:37 PM
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I know you don't want to hear this.... Go overdrive it is the best if both worlds. I took my car out with 3.73s on Saturday and man was it nice with over drive on the interstate. I tried 1:1 and the the 0.80 and 0.63 overdrive. It was awesome cruising at 80 like it was nothing.
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post #8 of 28 (permalink) Old Jan 21st, 20, 11:06 PM
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tacomatrd, what’s your RPM at 80, and what’s your engine dyno HP ?
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post #9 of 28 (permalink) Old Jan 22nd, 20, 12:14 AM
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Re: Rear Gear Recommendation

HI
VA Chevelle
without the diameter of the tire everybody is guessing !!!!

Engine rpm for 65mph is a good indicator 3spd auto/4 spd man top gear only
stock 2000rpm to 2500
modified street 2500 --3000
drag 3000-3300 little street
full drag 3500 ++
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post #10 of 28 (permalink) Old Jan 22nd, 20, 1:04 AM
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Quote:
Originally Posted by Reelysalty View Post
tacomatrd, what’s your RPM at 80, and what’s your engine dyno HP ?
Rpm at 80 is 2200
Engine dyno hp was 530 at the wheels at 5400rpm, falling around 500 at 6200.

I can cruise down to 1700 no problem now. After realizing I was blow my total timing and only running 25 at cruise I changed it so it comes in sooner and it made a pretty nice difference. Decided I may stick with the 3.73s now.
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post #11 of 28 (permalink) Old Jan 22nd, 20, 1:31 AM
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Re: Rear Gear Recommendation

Quote:
Originally Posted by VA64Chevelle View Post
Need recommendation on rear gear ratio for my 1964 Chevelle - 383 stroker motor @450HP , TH350 trans, 12 bolt rear. Car came with 4.26 rear gear.
Looking to change to a gear that will give be better cruising, but still with some bite. Would a 3.70 rear gear be a good option?
Thanks.
What series carrier? If 4 series you cant go numerical lower than 3.73, aftermarket makes a ”thin” 3.73 for 4 series carriers.
So you may be limited to 3.73, personally I think you will notice the difference but think it should run ok with 3.55.

How tall tires do you have in the back? Maybe 3.73 of you are limited to that and a taller tire.

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 #12 of 28 (permalink) Old Jan 22nd, 20, 9:09 AM Thread Starter
 
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Re: Rear Gear Recommendation

Thanks. I don't know which series carrier it has. Rear tires are P235/60 R15.
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post #13 of 28 (permalink) Old Jan 22nd, 20, 10:37 AM
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Re: Rear Gear Recommendation

That tire is about 26" tall (pretty short). A 275 60 R15 is 28". I would get happy with the wheel/tire you want to run before picking gears.

With your tire, 80mph would be about 4000rpms with a 3.73 gear. This would drop about 500rpms with a 28" tall tire.

If this is a driver, top speed and cruising/highway rpm is important. I have 28" tall tires with a 3.55 TH400 and it seems like a good compromise given no overdrive. Overdrive seems to be a great option if you want to run more rear gear.

69 Chevelle Malibu
427 sbc, TH400, 3.55 12-bolt
n/a best: 11.24sec/118mph
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post #14 of 28 (permalink) Old Jan 22nd, 20, 10:44 AM
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If you drive/cruise a lot on freeway etc, OD. Don’t even think about it. Going through the same on my ‘66 now.
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post #15 of 28 (permalink) Old Jan 22nd, 20, 10:52 AM
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Quote:
Originally Posted by tacomatrd View Post
Quote:
Originally Posted by Reelysalty View Post
tacomatrd, what’s your RPM at 80, and what’s your engine dyno HP ?
Rpm at 80 is 2200
Engine dyno hp was 530 at the wheels at 5400rpm, falling around 500 at 6200.

I can cruise down to 1700 no problem now. After realizing I was blow my total timing and only running 25 at cruise I changed it so it comes in sooner and it made a pretty nice difference. Decided I may stick with the 3.73s now.
The fact that your engine runs out of breath at 5400, combined with its’ ability to chug along 1700 at 80 along w/your recent thread considering changing cams for some pep tells me you have a purpose built very timid performer.

I have an engine which I believe is similar to OP. If so, a spirited, neck snapping performer. No way will it run under 2200-2400 at 60 or 80 @ 1:1 much less w/OD.

You do others a dis-service when you post only half the story.

BTW, I know of no such “engine dyno...at the wheels”. If you have an engine dyno report, what is its’ efficiency at say 5000 ?
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