Best rear axle gear ratio for 6-speed manual trans
Thread Starter
Senior Member
Joined: Mar 2014
Posts: 760
Likes: 10
From: NC
Car: 1986 IROC Z-28
Engine: 383 Gen I SBC 11:1
Transmission: Rebuilt TH700R4 for 500+HP
Axle/Gears: 4th Gen 10 bolt Eaton Truetrac 3.23
Best rear axle gear ratio for 6-speed manual trans
Hello 3d gen,
The machine I am modifying is a 1986 IROC Z28. The gracious owner who sold it to me for $500, made me an additional offer to purchase the original 1986 Chevrolet Camaro Service manual for an additional $40. I bit. Now they're mine!
This IROC has a GU5 RPO code which indicates a 3.23 real axle gear ratio on a 7.625" (7-5/8") ring gear. It accelerates great with the original parts.
But I wish to modify the entire vehicle while keeping MPG's as a priority up there with the HP. I mean it to be a street 'legal' machine.
The first mod will be the cheapest which would be the rear diff.
Also, it would be nice to swap out an entire drum brake assembly with disc brake assembly. But this has been difficult to find.
So, now I'm currently looking at gearsets on JEGs and to keep the drum differential.
I've found a 3.73 gearset on JEGs for <$200.
Some other notes:
The trans is a automatic TH700R4, but eventually it will be a 6-speed manual.
The engine is the original 305CI, but I aim for a 383CI w/~450 HP.
The machine I am modifying is a 1986 IROC Z28. The gracious owner who sold it to me for $500, made me an additional offer to purchase the original 1986 Chevrolet Camaro Service manual for an additional $40. I bit. Now they're mine!
This IROC has a GU5 RPO code which indicates a 3.23 real axle gear ratio on a 7.625" (7-5/8") ring gear. It accelerates great with the original parts.
But I wish to modify the entire vehicle while keeping MPG's as a priority up there with the HP. I mean it to be a street 'legal' machine.
The first mod will be the cheapest which would be the rear diff.
Also, it would be nice to swap out an entire drum brake assembly with disc brake assembly. But this has been difficult to find.
So, now I'm currently looking at gearsets on JEGs and to keep the drum differential.
I've found a 3.73 gearset on JEGs for <$200.
Some other notes:
The trans is a automatic TH700R4, but eventually it will be a 6-speed manual.
The engine is the original 305CI, but I aim for a 383CI w/~450 HP.
Last edited by UltRoadWarrior9; Mar 26, 2014 at 06:10 PM.
Joined: Apr 2005
Posts: 7,736
Likes: 14
From: Not in Kansas anymore
Car: 82 Z28
Engine: 383 SP EFI/ 4150 TB
Transmission: T400
Axle/Gears: QP 9" 3.73
Re: Best rear axle gear ratio for 6-speed manual trans
Joined: Jul 2000
Posts: 6,529
Likes: 94
From: Aridzona
Car: `86 SS / `87 SS
Engine: L69 w/ TPI on top / 305 4bbl
Transmission: `95 T56 \ `88 200-4R
Re: Best rear axle gear ratio for 6-speed manual trans
Wait until you do the 6 speed and 383 to make the change and definitely do it then.
The 3.23 ratio won't work well with a 2.66 / 0.50 T56. You should go 3.90 rear gears with that.
And 3.73 or 3.90 or 4.10 gears aren't ideal for the 305/700-R4.
Furthermore, a 2.66 / 0.50 T56 and 3.73 isn't a great match for the 305.
Make it a matched drivetrain by saving your money and wait to do the mods instead of mismatching things over time.
The 3.23 ratio won't work well with a 2.66 / 0.50 T56. You should go 3.90 rear gears with that.
And 3.73 or 3.90 or 4.10 gears aren't ideal for the 305/700-R4.
Furthermore, a 2.66 / 0.50 T56 and 3.73 isn't a great match for the 305.
Make it a matched drivetrain by saving your money and wait to do the mods instead of mismatching things over time.
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