projectheaven

Hello all,
Maybe someone could clear up the confusion for me.I'm going to throw a posi unit in my 72 olds and I'm not totally sure what I have under there.I've heard its the corp. 8.5 and 28 spline.Would there be a reason to think other wise?
Also I want to put 3.73's out back but what's the differnce between those and let's say 4.11's? Are those freeway gears and 3.73's help get you off the line faster?

dan2286

4.11's will help you get off of the line faster, but raise RPM's on the highway. 3.73's will be slower off of the line but will lower RPM's on the highway.

To be able to help you pick the best gear ratio, we need to know a lot more info about your engine, trans, and tire size. Are you planning on putting a 455 in the car? That will also make a huge difference on what gear will be the best.

lshlsh2

Freeway gears would be like 2.56 to 3.08

They would keep your rpms down at speed improving gas milage but are not good for getting off the line quick or racing

projectheaven

I just bought a 455 that I'm planning on getting rebuilt and throwing in,in the next few months.I'm either going with the 2004r or the 700r4 and my tire size is 24 inches.
I'm not going to be racing it,mainly just a show car but I want that power to be there if needed!

MN71W30

If you don't plan to race I would stay around the 3.31 to 3.42 range. They are good enough gears to get you down the drag strip if needed and to cruise at 75 miles per hour easily with the the overdrive tranny you'll be running. I have had just about every street ratio and I set up rear ends and the older I get the more I like 3.31 gears. If you run 373 gears in your car first gear will be very short and annoying after a while, especially with a shift kit.
So in closing my humble opinion would be 323,331,342.
You are definatly on the right track with the OD tranny.

Why don't you calculate the rpms with a formula to be sure of what your doing

MN71W30

MN71W30

[ and my tire size is 24 inches.

It looks to me like your rims might be 24 in. You need the total height of the wheel and tire diameter for the formula
And also remember your overdrive drops the rpms even lower. You'll have to find out what the od ratio is.

projectheaven

Wow! That is great info.I'm definitely going to put that to use. I'll keep you posted.

monzaz

overdrive multiple by .78 on most trannies ...so if you have 24" tires and the rpm is 3000 rpms at 60 then take 3000 x .78 =2340 and then some trannies lock to knock off another 200-300 rpms...and it will give you the new rpm ...remember that auto transmissions have some slip to them always unless you have a lock up convertor on the over drive trans.

If you need a quote on anything rear diff give us a shot then you can compare else where too. Jim

J D Race.com