Rear end ratios

[Guest bofusmosby]

There has been a lot of talk over the years about the different rear end rations in our cars, and I know that for my car, there were actually 3 available for my 37 Pontiac. There is a listing on Ebay

1937-38 OLDSMOBILE, BUICK, PONTIAC 6 & 8 CYL AND OTHER MAKES RING GEAR & PINION | eBay

showing these "ratio" gears, and I was wondering if these would be the ones that would need to be replaced to accomplish this task, or are there any other changes that would need to be made? I'm not saying that I intend to buy these now, but in the future, a good possibility. I believe that my car (from Montana) has the mountain gear ratio, and living in Florida, this is a bit much.

Thank you in advance for any and all comments.

[Coley]

What is your "mountain gear ratio". How many and what were the options for your car.

The ring and pinion gear set is what you would need to change. BUT...

Some ring gear carriers only take one ratio and may need to be changed to go to a lower ratio (higher speed).

A lot of confusion goes along with gear ration discussions.

[NTX5467]

Usually, as in more recent GM rear axles, the part of the differential to which the ring gear bolts has a particular thickness, which relates to the "build thickness" of the two items bolted together AND the diameter (and number of teeth) of the pinion gear. Regardless of which gear ratio is in the car now, it's that build thickness which must be maintained.

If, a more recent GM rear axle has a current ratio of 2.56, then to get to a 3.73, it takes a spacer between the new ring gear and existing differential flange to maintain the total thickness of the assembled differential and ring gear. Similar with going to a 4-series ratio, too. Usually, though, the aftermarket suppliers of these deeper gears provide the spacers as a part of their gearset package, otherwise, it would take a new differential designed/machined to take the particular deeper gear ratio.

Best thing to do would be to, if possible, find a vintage parts book for the model year and brand of your vehicle. Then look in there to see what gear ratios the particular differential housings will tolerate. This will also nail down the optional rear axle ratios OR rear axle ratios in particular models and engine/transmission combinations, too.

Usually, the ring gear/pinion gear "teeth" numbers are stamped into the outer surface of the ring gear. They are a matched set and should be considered such.

But there was one time, two friends of mine discovered the difference between a GM 3.08 gear set and a deeper 4-something gear set was ONE less tooth on the pinion gear. A clandestine activity then progressed to exchange one pinion gear (in a "friend's" rear axle with that particular pinion gear) with the one in the 3.08 axle. Worked decently well enough to last a good while, surprisingly.

In the world of vintage Buicks, the "center section" of the rear axle will interchange for many years in the '40s - earlier '50s. In that time, the later ones had more "high speed" ratios than the earlier ones. Changing the complete center section is easier to do that changing (and setting up!) the ring gear sets, so many people have swapped the later center sections into their earlier rear axles to make the cars more useable in more modern "higher road speed" times.

Just some thoughts,

NTX5467

[Curti]

I would determine if the rear diff is the same as Chevrolet; I'll bet it is. I once swapped out an early Chevy for a complete axel assembly from a 50's power glide car. If my memory serves me the glide was close to 3.25 as opposed to the 4. gears in the early car

It was about a three hour job. It made a world of difference the way the car drove.

[Guest bofusmosby]

Thank you for your replies. My 37 Pontiac originally came with 3 different ratios in the rear end. One was a lower ration, and designed for mountain driving. The middle one was for hills, and a mixture of both. The 3rd was for flat lands/plains. My car came from Montana in the mountain areas. I am assuming that the ratio on mine was the mountains ratio. I was told by a fellow 37 Pontiac owner about this, and I was also told that taking his car 55-60-mph was not a problem with his car. Well, after my car gets up to about 45-50mph, the RPMs are so high, I'd rather not take it to 55-60MPH. I believe that this would be too much of a strain on the engine. This is why I believe that my car has the mountain ratio. When I saw this on Ebay, I just wondered if these were the parts that determined the rear-end ratios, and if there were any other parts needed to accomplish this. I do not plan on buying these at this time, but if this is all I need, then I'll know what to look for in the future.

[NTX5467]

When you take the differential assy out, it also opens up possibilities to also change the two "side bearings", plus inner and outer pinion bearings. Of course, a new "crush sleeve" should be in the mix, too, plus a new pinion seal. Might also need a new pinion nut, too? AND it also makes for an opportune time to do any replacement of the driveshaft u-joints. And, with the driveshaft out, that also leaves the transmission extension housing seal and bushing as "easy pickings" to change, too.

As they say, "One thing leads to another" . . . and "While you're in the neighborhood . . ."

Enjoy!

NTX5467