1939 Buick rear axle ratios

[Jon Robinson]

Hello. I have a question about the rear axle ratios on a 1939 Buick Special.

I've been restoring Chryslers and DeSotos for nearly 25 years, and I drive my 1950 DeSoto Custom club coupe about 10,000 miles a year, so I'm a real believer.

I'm not exclusive, though, and I have a great deal of affection for straight-eight Buicks, and I've enjoyed every one of them I've driven. I've got my eye on a 1939 Buick Special sedan, and I know that rear axle ratio has a greater effect on the way a car drives than which engine it has, in some cases.

Please tell me the rear axle ratio on the 1939 Special. Being that it's a comaparively light car, I'm hoping it's down in the 3s -- like 3.90 or so. If so, I'll keep the speed down around 55 MPH, and it'll be fine, I'm sure. If it's something like 4.11 or 4.30, I'd be concerned. We had a lot of good cars before 1955 or so, but the roads were terrible, and some really good old cars are hamstrung by low-geared rear ends.

Part Two of this question is what later Buick rear end could be harmlessly installed without ruining the originality of the car. For example, guys with early-1950s standard-shift Chevrolets often install a rear end from a Powerglide car of the same era to lower the RPM about 20%. On my Chrysler products, it's really easy to change just the differential so you wind up with the same rear end in the car with only a different number of teeth on the gears. My DeSoto went from being a 55-MPH car to being a 65-MPH car when I installed a 1957 Dodge rear end that took the ratio down from 3.90 to 3.36. People are amazed at how fast this supposedly slow Fluid Drive DeSoto is, and I have witnesses to my 20-23 MPG under good circumstances, mostly meaning no hard head-winds.

I'm a good mechanic, and I'm familiar with the enclosed driveshafts on the Buicks, and of course, I want to keep that original equipment. I would only consider changing the ratio if I could do it by, for example, installing a 1949 Buick Super differential from a Dynaflow car which had a ratio about 15% better than a standard-shift '49 Super.

The low-geared rear ends in these cars were only forced by the bad roads of the time, and many prewar cars went to the junkyards only because the rear axle ratos were too low for the 1950s-'60s multi-lane roads. I view changing to a later-but-CORRECT rear end as a step toward preserving original engines -- especially these long-stroke engines which take a beating from high RPM.

Thank you in advance for any advice you gearheads may have. I do the same for a lot of Chrysler guys (and girls, too). I may join the ranks of Buick Eight owners soon.

JON

[The Old Guy]

The 39 Special had either a 4.1 0r 4.4 ratio standard. there was a 3.9 available in the Century and as an option in the Special. If you can find a 3.9 you will be lucky, as many others have already pretty much used up the supply. GM made a change in 1940, and the rear ends from 1940 to 1955 would interchange. I have a 3.4 in my 40 ,and it makes the car able to cruise the interstates with no problem.

[Jon Robinson]

Old Guy:

Thanks for the info. This is exactly the kind of expert information that I was looking for.

Just out of curiosoty, what else would one have to change to make the 1940 and later 3.40 rear end work? Would you have to change the whole transmission, or could you get away with just changing something like the torque ball clamp?

If it's something small, I would consider changing it, but if it's a big alteration of the car, I'll pass up this car. I would rather pass up the car than change it very much because I'm such a believer in originality.

I live in the Mojave Desert in southern California, and I routinely drive 100 miles between towns. I wouldn't want to beat on this Buick by pushing it at 60 MPH for hours with a 4.11 rear end. I'm actively looking for a prewar sedan, but I might just have to stick with my Chryslers and DeSotos with overdrive because of where and how much I drive.

Thanks for the info, and if you have any suggestions on how to harmlessly make a later 3.40 Buick rear end fit the car, I'd be very interested.

JON

[Guest AllBuicks]

Hi Jon,

I had a 41 Super (56C) with a 4.4 rear end. I loved the car, but it was near impossible to run it on the highway at a safe enough speed where you wouldn't get run over. Many old timers told me I just didn't know how to drive the thing and that in their day they would drive them at 70+ down the highway!

I am currently restoring a 38 Special (40C) that has a 248 with a 4.4 ratio. I am sure this will be just like my 41. The 37/38 Buick Club prints a real nice magazine every few months that has a good technical section. In the past issue there is an article about swaping the rear end from a 55 to a 38 which took the ratio down to 3.36. This does require some machining and welding but I would imagine the end result would be well worth the time and money. The article says that rear ends from about 1941 thru 1955 can be swapped to prewar cars with these modifications. If your interested in the article I can either fax it or scan and email it to you next Tuesday.

Glenn

[Jon Robinson]

Thanks, Glenn. I would be very interested in seeing the article.

I have a fast internet connection, so go ahead and scan it, and e-mail it to me:

jrobinson@uia.net

It would be best for me to stick with the Chryslers and DeSotos I'm familiar with because, in addition to my knowledge of them, I also have extra correct engines and other parts, but I've seen a Buick Eight in my future many times, and I really like this '39. The information you send me might help me make up my mind, so even if I pass on the car, I'll appreciate your help very much, and I'm always interested in learning about all old cars anyway.

Thanks!

JON

[Guffin]

Please, e-mail it to me too.

jan@specialinstrument.se

Thank you!

[Matt Harwood]

I'd love to see a copy too! Please add me to the list!

toolman8@sbcglobal.net

Thanks!