[buisoma]

I have a 1968 chevelle 396 with a muncie 4 speed transmission, 12 bold rear 411 gears with positraction. I would like to change my gear ratio to something around 370 so i can cruise the highway a little better. My question is how hard of a job is it. I'm looking for imput so i don,t take on something i might regret.

[The Factory Man]

Originally Posted by buisoma

I have a 1968 chevelle 396 with a muncie 4 speed transmission, 12 bold rear 411 gears with positraction. I would like to change my gear ratio to something around 370 so i can cruise the highway a little better. My question is how hard of a job is it. I'm looking for imput so i don,t take on something i might regret.

That would be a 12 bolT? Changing differential gears and doing it correctly is not something many people are capable of. Here is my iNput:
Do you know how to read and adjust the gear pattern?
Do you have a dial indicator for reading the backlash?
Do you have a 0.00 to 1.00 inch micrometer to measure shims?
Do you have a pound/in torque wrench for measuring pinion torque when adjusting pinion pre-load?
Do you have a lb/ft torque wrench?
How are you going to hold the companion flange while crushing the crush sleeve?
These are just some of the things to consider. I don't consider a ring and pinion change a hard job, when I was making a living as a tech it was a job I liked to do and I have done many of them.

[big gear head]

I agree, setting up a ring & pinion set requires some special skills other than normal mechanics skills. I have coached many people through this on the internet, but it takes time and a lot of patients.

You need to find out if you have a 3 or 4 series posi in the car. If you have a 4 series then you are not going to be able to go past a 3.73 gear. There is not much difference between a 4.11 and a 3.73, so it might not be worth the trouble to change it. If you want to see a difference then go to something around 3.42.

[buisoma]

how do i find out if i have a 3or 4 series posi

[big gear head]

Remove the cover and look for a casting number on the differential case. If it's an Eaton Posi then it will have the number EDB30140 on a 3 series and EDB30174 on a 4 series. If it isn't an Eaton then look for a ring gear spacer or a thick ring gear.