Rear diff ratio recommendations

[keith4909]

1967 Gto
474 roller motor ported cast iron heads
All the aftermarket goodies
600lb tq
450-480hp
Turbo 400
275/35r18 rear tires

Purely street. No drag

Looking to have a 12 bolt built. I was thinking 3.27-3.55. I'm told my 10 bolt will break.

[andrewb70]

Maybe even 3.08...that's a pretty short rear tire. Even with the 3.08 you will be at over 3000 RPM at 75 MPH.

Andrew

[nokones]

If you are just grocery getting in town the 3.27 to 3.55 will probably feel pretty good with a Turbo 400. The Turbo 400 first and second gears are a little on the tall side. I don't think you will be happy with a 3.08. Now, if you going to cruise a lot on the Freeway, you'll probably be happier with a 3.08.

[cactuss4]

thanks for asking, want to swap out my 4.11's on my BB 396 4spd Chevelle. I was thinking 3.55's but I want something that will work with the T56 when that eventually happens.

[keith4909]

I think the car has 3.30 ish. Running 3k at 65/70 mph. If I go with 3.08 will the car feel like a pig off the line?

[nokones]

I think the car has 3.30 ish. Running 3k at 65/70 mph. If I go with 3.08 will the car feel like a pig off the line?

It may with the tall Turbo 400 first and second gears. But, on the other hand you may be ok with that amount of torque and if your cam is profiled for low-end torque. A good tranny shop that knows how to work on Turbo 400s may know how to change your planetary gear set so the first and second gear ratios will be on the shorter side like a 700R4 first and second gear ratios. The 700R4 first and second gear ratios are 3.07 and 1.62, about a little over of 20% of gear reduction. With those gears you will be alot quicker off the line. The only problem you need to be aware of, the pinion gears in the planetary gear sets are real small in order to get the lower gear reduction and they usually won't handle a lot of torque. You will break a lot of gear sets. I ended up going back to the standard planetary gear set to handle the torque.

[68Formula]

I think the car has 3.30 ish. Running 3k at 65/70 mph. If I go with 3.08 will the car feel like a pig off the line?

With that much low-end torque, you probably won't miss the 3.36s much, and you'll certainly appreciate 3.08s on the highway.

[keith4909]

Just found a complete rebuilt 12 bolt posi in SoCal with 3.31's for 1600.00. Year warranty and the axle width is the same. I can go with a moser unit for 3400.00 so this is a steal. Putting a deposit through PayPal and I will drive down from NorCal to get in the next week or two.

[keith4909]

My original SoCal deal feel through. Found a 1965 chevelle 12 bolt 3.07 posi with new moser cover, axles, Wilwood dynalites, brake hoses, upper arms and e brake cables. Paid 1600 plus freight. Very happy with the rear end. Great gear for my build