Change rear end gear ratio

[Don kelley]

I have a 1969 corvette with 4.11 to 1 gear ratio(code AP) rear end. I would like to change the gear ratio to a 3.08 or so. Currently I just don't take the car on highway trips because I don't like runnig the higher RPM for longer trips. Not to mention the gass mileage.

Can I just change the ring & pinion gears or is there more to it? If thats all I need to change, since my ring gear has 37 teeth, can I just change out my pinion to 12 teeth for a 3.08: 1 and use my ring gear or are they matched sets?

[Dick W.]

Re: Change rear end gear ratio

Ring gear and pinions are used as a matched set. You cannot mix and match. You will also have to use a different differential case as the higher numerical ratios (4.11 & 4.56) use one different that the lower numerical units do. It would be cheaper and easier to purchase a complete differential than to convert yours.

[Roger Legge]

Re: Change rear end gear ratio

I guess I've never thought about it relative to a C2/C3 IRS, but do they have different carriers like a 10-bolt or 12-bolt GM rear diff? The 10-bolt and 12-bolts have '3 series' and '4 series' carriers. If one wanted to go from a 4.11 to a 3.08, you'd need to switch to a 3 series carrier.

Roger

[Dick W.]

Re: Change rear end gear ratio

Same difference in Vette rears.

[Joe L.]

Re: Change rear end gear ratio

Don-----

As Dick mentioned, to change the gear ratio, you have to change the ring and pinion as a SET and you also have to change the positraction carrier case. Those parts plus the labor to do the rebuild/change will be substantial.

However, you can use any differential unit from a 1963 to a 1979, although I recommend using a 67+. During the 70's there were lots of Corvettes manufactured with 3.08:1 differentials and many of those cars have gone on to their "final reward". So, there are plenty of these units around. Many were originally used in the lower powered 1970's Corvettes and behind automatic transmissions. Most of those should be in pretty good shape.

[Joe L.]

Addendum

Just to be clear about it, the differentials used on any 67-79 Corvette will be equally "strong" as your 1969 unit. In fact, they will be EXACTLY the same with respect to all aspects of internal and external components and all 1970 and later are positraction. The only differences are the side yoke configuration (which can be easily changed from cap-type to u-bolt type and vice-versa) and the differential COVER used on 78-79 (you can use your original cover to replace this perfectly)

[Tom S.]

Re: Addendum

Don
I just changed the 4:11 in my 69 to a 3:36. I bought a good used one for $350.00. I am very happy with the out come. You will have to change the drive gear and the driven gear in the tranny to get the correct reading. I have saved the 4: 11 which was in good cond. in case I sell the car. What a difference in road driving, like a new car. What trans. do you have? I have an M-21, and I would not want to go much higher than what I have because of the higher 1st gear in the M 21. If YOU HAVE AN M 20 a 3:08 would work very well.

Tom