I have a 1970 cutlass W-31, It has a 3.91 rear end with the 3 spd auto trans. What would be a good gear to put in rear end if i'm putting 275/45/18 wheels and tires on it. It screams on the highway currently, so I want to make it alittle more highway friendly with out losing to much snap! Thanks!

 

Your only option is 3.42s if you want to reuse your existing posi carrier. Also they must be OEM 3.42s as the Richmond 3.42s are designed for the 3.08-3.23 carrier.


If you find OEM 3.42s & want to sell your 3.91s & they are in good shape with a nice pattern I would be interested in buying them. Shipping would be to Sweetgrass MT 59484 for p/u @ the border.

 

Measure the diameter of the tires you intend to run on the rear, find your target cruise rpm, and plug the numbers into the rpm calculator. Usually anywhere between 2100-2300 rpm @65/70 mph is comfortable cruising for many folks.


http://www.csgnetwork.com/multirpmcalc.html

 

If the OP really has a W-31, then he should not follow this advice. With a tire diameter approaching 28", he would need 2.56 gears to get the RPMs that low. That cam with those gears would be a major dog. Woof.

It's worth noting that in 1970, the standard rear end for a W-31 was 3.91, and the factory wouldn't even let you order anything lower (numerically) than 3.42.

Franky, if you want low RPM on the highway you shouldn't be driving a W-31.

 

Before you spend any money on a perfectly good rearend(assuming) You may want to consider a 200 4r 4speed. It may pay for itself if you drive it much. Beefed up would be good.

 

I bet funkwagon meant 3100-3300, which is pretty close to the RPM's you will see with that tire/rear end combo at ~65MPH. No big deal other than MPG unless you do a lot of highway miles at higher speeds. Enjoy it for what it is (a freakin' W31!), get an OD tranny or start looking for a SX/turnpike cruiser.

 

Actually I did mean 2100/2300, I am spoiled with overdrives though. BlackGold's comments are merited, I just have the tendency to factor the 0.68 into my personal rpm comfort zone. Although a 3:08 would drastically help the drivability on the highway, the cost and availability of the gears and carrier would be significant financially as well as effort. Personally before I altered the gear arrangement, I think that (like dmullin) the overdrive would be the easiest solution to "have your cake and eat it too". Just store the original gearbox and driveshaft for originality's sake. I have a 3:08 in my '70 as well as a fresh 3:42 ready to install, but am waiting to have a 700R4 built before I do the rear swap. To each his own..