rear end gear - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 09-19-2018, 11:06 AM Thread Starter
 
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rear end gear

I have a 1964 GTO 389 tri power with a Muncie 20 close ratio 4 sp. I have 4:11 rear end. can I drop to 3:56 or 3:36 to lower my rpm?
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post #2 of 6 (permalink) Old 09-19-2018, 11:23 AM
 
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Welcome

welcome

a few questions arise ........

I am no expert .. but been around these cars for 40 years....

is the differential the original 10 bolt ? 411 sounds chevy ish we had 390 and 433

yes a gear change is a great way to lower rpm

but

if you do have a close ratio m 21 going lower than 355 the ratio between 1st gear and
the differntial makes for a chattery clutch and lack luster performance off idle
and starting on a hill ... well ... its not fun ...or easy on clutches...

if its an m20 wide ratio alot better .... for the 336 ratio

355 and close ratio I dont think ever came together in our gto's

closeratio needed a 390 gear to get rpm's up

there is alot of good info here .....

ole pinion should chime in ,,, I havent seen him on in a while .. hopefully hes fine ..

basically you need to know for sure what your first gear ratio is

for starters probably a 325 case close ratio and 390 gears

even wide ratios with the lower first and second gears and 355 wind out pretty good

with the stock 14" tires ..... whats your rpm on the freeway at 60 and tire size ??

is it an aftermarket or original tach .... many original tachs need calibrated and repaired

DO NOT TRUST AN ORIGINAL TACH for accuraccy until you have put a known modern one

hooked up along side to compare on a run around the block.....


these are just my opinions and insight from tinkering on over 100 pontiacs in my auto maintenence life



welcome aboard

ScottT
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post #3 of 6 (permalink) Old 09-19-2018, 01:28 PM
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If an 8.2 ten bolt (stock) you can drop in gears as low as 3.36. I did it with my '65 28 years ago and it transformed the car. Go for it.
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post #4 of 6 (permalink) Old 09-19-2018, 03:14 PM
 
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"...411 sounds chevy ish we had 390 and 433..."

Yeah, the 3.90 & 4.33 was available from the factory. But, there have been aftermarket 4.11 gears for the 8.2" BOP.

Here's some 4.11's at Summit.

https://www.summitracing.com/parts/m...2411/overview/

Here's some cheap ones on Ebay.

https://www.ebay.com/itm/GM-8-2-BOP-...-/370875730118

So, the car in question could easily have aftermarket 8.2 BOP 4.11 gears.

"...can I drop to 3:56 or 3:36 to lower my rpm?..."

3.36 & 3.55 gears are still sold new.

The factory 8.2" BOP rears had a cover like shown in the pic below.
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Last edited by bigD; 09-19-2018 at 03:28 PM.
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post #5 of 6 (permalink) Old 09-19-2018, 05:30 PM
 
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Mine came with 3:55 as stock, still to much rpm for highway ([email protected]). Found a 4:11 and took quite a while to swap it in and get the spacers correct etc, but once done the swap was easy, new crush washer everytime, about 1.5 hrs. Used the 4:11 while at the drag strip, 3:55 for every thing else.
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post #6 of 6 (permalink) Old 09-20-2018, 07:19 PM
 
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336

I have 336 gear in my 64 ,4 spd and I am pleased with them . I have found that original GM gears were less likely to whine than aftermarket sets . JMO
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO General Discussion

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