Rear end gears - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 09-02-2018, 12:08 PM Thread Starter
 
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Rear end gears

I have 2.78:1 rear end gears in my 69 lemans and I am trying to figure out what I'd need to bump it up to the 3.23-3.42 range. I am looking for better acceleration, but still something I can cruise 70mph with.

Is it true that the 3 series gears need a different carrier?
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post #2 of 6 (permalink) Old 09-02-2018, 04:43 PM
 
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Not sure about Pontiacs, but Chevys have a 3 series and 4 series rear. What transmission are you running?? To have a 3.42 gear and cruise nicely at 70 mph, you might require an overdrive auto trans. or a 5 speed manual trans. Just my opinion. I'm presently running a 3.55 gear in my '71 GTO with the factory TH400 trans. I normally don't drive it 70 mph and I try to avoid the interstates. I know the engine is working a little overtime at 70 mph. I like the 3.55 gear. Good for a little street light action. Gets you up and running. I drive for the most part in the 45-50 mph range. No rush for me. No real place to go.
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post #3 of 6 (permalink) Old 09-03-2018, 08:35 AM Thread Starter
 
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I was worried about cruising speed, but I would like better take off. The car is a th400 car, but I was thinking of swapping to a m20. That wouldn't make it faster off the line though..
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post #4 of 6 (permalink) Old 09-03-2018, 10:50 AM
 
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I think the 8.2 BOP carrier breaks are <2.93 / 2.93 - 3.23 / >3.23.

I changed the 2.78 gears in my '70 for a 3.36 gear set and upgraded to an Eaton posi at the same time. It turns ~3000rpm at 70mph with a TH350 and 245/60-15 tires. Not great, but bearable for shorter trips. It works for me, but I'm considering a 200-4R swap down the road for better highway cruising.
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post #5 of 6 (permalink) Old 09-05-2018, 09:14 PM
 
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I cruise at 90 with 3.23 all day long... Seriously, it's a good ratio for what you are after. I do very long runs at 80 or 90 with my 69 auto Turbo-Hydromatic GTO.

*67K miles... Under Construction* :)
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post #6 of 6 (permalink) Old 10-11-2018, 05:19 PM
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Yes, if you have a 2.78 gear, and want to run a 3.23, you will need the next series carrier which will accommodate 2.93, 3.08, and 3.23 gear ratios. The 2.56-2.78 carrier is totally different to accommodate the thinner ring gear and larger pinon gear these highway ratios use. If your car has an automatic trans, I personally would keep the 2.78. Great ratio for modern day speeds encountered every day.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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