Hi everyone. I have a 1970 olds cutlass S with the W-31 package. It currently has the 3.91 rear end with a turbo 350 trans. which is only a 3 speed with no overdrive. So here is my dilema, on the highway at 55mph it runs around 3000rpm and at 70 its around 5100 rpm. So its suck on the highway. I can get a 2004r with ovrdrive transmission from a friend but needs to be rebuilt. Or I can regear it for $500. What makes more sense for long term drive ability? And If I go with the new gears whats the best set to go with. A lot of people tell me 3.08 or 3.23. I don't know much about gears. Any help will be appriceated. Thanks

 

I don't think your tach is correct. Your rpm numbers don't work even with a 26" tire, you should only be running around 3500 rpm. Here's a calculator.


http://www.angelfire.com/fl/procrastination/rear.html

 

If i'm reading the calculator right, this is the speed it showed me:: The speed in MPH is 55.9237 miles per hour.
The tire and wheel size I have on there is 225/65/14, the stock size from factory is 225/70/14.

 

The trans swap is preferable to the gear swap. Right now your effective ratio in first (for off-the-line acceleration) is 9.85:1 (3.91 x 2.52 first gear in the TH350). Changing to even a 3.23 (which will require a different carrier in your rear axle) yields an effective 8.14:1 in first (and obviously 3.23:1 in third).

With a 200-4R and your current 3.91 gears, you get an effective 10.71:1 in first, for better off-the-line acceleration, and an effective 2.66:1 in O.D. for highway cruising.

Just keep in mind that you'll spend about three times as much money for a properly built 200-4R than on the gear change.

 

3.42 gears will work with your existing carrier and were optionally available with the W-31 package. With a W-31 engine you will hate the low RPM performance with anything less than 3.42 gears. If you run a 3.08 or a 3.23 gear set you will also need to change the carrier.

The easiest solution is to switch to 15" rims and put a tall tire on the car. This will reduce your effective drive ratio and afford lower rpms while cruising. A 275 60 15 fills the rear wheel well and looks great. It is 28" tall on an 8" rim.

IF you choose to swap out your 3.91 gear set I would buy them from you. My w-31 had OEM 3.91s but has 3.42s in it now.

please post your location in your profile and tell us more about your W machine and especially post more pics.

 

A third option would allow you to keep both the 391s and the TH350...google gear vendors over drives. As Joe mentions you’d need to spend a bit to get a TH200s durability/longevity to equal the TH350 behind your W31.

 

You should be turning 2835 rpm with 225-65-14 (25.52" diameter) @55 mph. If you are going 70 mph your rpm will be around 3610, not the 5100 you expressed in your first post. The stock rear tire would have been just a tad bigger in diameter (.88) 26.4". That would lower your rpms to 3485 @ 70 mph, and 2740 @ 55 mph.


The most bang for your buck, even though the initial expense is a lot higher, is the trans change, .67 in OD. This way you will still have the off the line performance but it would cut your cruising rpm considerably to 1900 rpm @ 55 and 2415 rpm @ 70.

 

Unfortunately, you'll spend even more on a GV setup than on a properly built 200-4R, especially once you factor in the custom driveshaft.

 

You should double check you tire size. I don't think this even exists

225/65/14

 

Yes Sir slightly true but you get to obtain originality of the trans and rear if thats a goal? Which it certianly would be for me with a W car. Though I suppose all of this is bolt on parts. The reason Id go in this direction is simply personal as Ive seen the guts of a 200 (GN) compaired to 350.
A GV OD is ~$2700(last I looked) plus a shortened shaft or a custom shaft will be 1-300. Friend just purchased a built 200 from Art Carr for just over 3K so its about apples to apples.

 

Let's not forget that this is a W-31 and do anything silly to it. The cheapest over all solution is to change the rear tire to a 225 70 14 and drive that car the way God and Dr Olds intented.

 

I just purchased a GV overdrive $2495. shipped to my door. I had estimates of 3000 for a 200R4 plus shipping. My OD will be behind my original Turbo400. GearVendors OD is rated to handle 1200 HP and has a 2 year guarantee even if you race it. I dont know but i dont think the 200 builders will guarantee their trans the same. I also have a 3.90 rear. I think my final drive ratio will be like a 3.04 and its basically a 6 speed with the GV OD. I will have to have my driveshaft shortened some im hoping no more than 150 for that but i dont know yet.

 

While you will have six speeds, two of them are redundant. The advantage of the 200-4R is that you get a lower first gear AND a higher high gear. The GV only gives you the higher high gear. Your first gear effective ratio is unchanged. Splitting first and second is pretty worthless unless you are pulling a very heavy trailer.

 

Amen!!! If you don't like high RPMs, then you shouldn't be driving a W-31.

 

I use a 2004R with a 3:90 gear in my 65 it works quite well, I have close to the HP rating on the W-31, I spent about $2500 having my transmission built.
I think the OD transmission is worth exert penny it cost.