The Stovebolt Site Forums The Parking Lot Big Bolts checking gear ratio

in another thead i posted about doing a rear swap on my 42 ton and a half i have...ive been thinking about it and have decided to leave the factory 2 speed under the truck if the gears are not to steep..ive locted a later model wheel to swap tp a 19.5 from the factory 20 if anybody is interested the wheel numbers are for steel accuride wheels #29667 and in alcoa aluminum# 76100..these wheels are made for p-30 box vans and they bolt right up...but any way is there a way to check gear ratio without pulling hoghead ?is there a number casted on the housing some where please give me all info you can ...thanks for all your help

Before splurging on those wheels, your will need to make sure they clear the drums on the rear.

There has been a fair amount of discussion here about it and the final determination was that "It fits on some trucks and not on others, you have to try your individual truck."

I haven't tried on my 39 yet. I forget who tried but didn't fit, but I am sure they will chime in soon.

Do a search and you will find plenty on how to check your ratio.

Its a pretty good chance your high is about 6.17:1 and your low is about 8.10:1. This is about a 30% jump (or drop) essentially splitting 3 and 4 on the granny 4 speed. Many people will say it is a low speed to help you get a load moving (and that is how I normally use it), but in fact it also allows you to drop to 4 low instead of shifting down to third when climbing a hill (I use it for this also, but I'm not as good at the downshift as I am at the upshift). Hope that made sense. The 2 speed rear allows you to split gears to stay in a better power band and keep rpms down, but it takes some practice to do it.

That would be me. I hauled home all 6 wheel and tire combo's just to find out they wouldn't fit over my rear drums. Others have done it with no problems. Borrow one before spending the money,
Scott

Low54, I think if you raise the rear wheels, and count the number of revolutions of the drive shaft, to complete one revolution of the wheel, that will tell you your gear ratio. I have not personally done this, but it seems like that should work. If I am correct in this assumption, you would have to do this in both high and low.

The more accurate way that you devise to measure wheel and drive shaft movement, would determine how precise your results would be. I would think that a mark from a paint marker, and screwdriver clamped a pair of vise grips for the pointer would be sufficient.


Has anybody ever tried this?

I don't think you would be able to rely on any casting numbers, as there are probably a lot of gear combinations that could be interchanged in the same housing.

Let me know good or bad if this works, as it almost sounds to simple.

John

That's basically the way it's done.Be sure to only jack up one wheel.I usually use yellow tire crayon and mark a place I can see on the backing plate and the rim at the same place,then mark the pinion yoke and axle housing in the same manner.then just rotate the wheel slowly and count the revolutions of the drivelineto make one full revolution of the wheel.If the driveline finishes on a partial turn,try to round it off to the nearest 1/8th turn,for accuracy. That'll give you your gear ratio.

http://www.angelfire.com/fl4/pontiacdude428/Hot.html
http://www.onlineconversion.com/bigger_tires.html

These are a couple of good nomber crunchers to figure out how tire sizes or gear ratio changes will affect your road speed,as well as other things you've always wondered how to figure out.
Speed

Jacking up one wheel, you must count the driveshaft revolutions with two turns of the wheel to get the ratio due to the action of the differential gears.

An original '42 2-spd rear would have ratios of 5.64:1 and 8.22:1. If you can find the serial number stamped on top of the unit just ahead of the shift lever, it should have a prefix of 2AB or BV, if it's original.

Ooh, I need one of those! My 39 has just less than 6.25 on the high end and just over 8 on the low end. 5.64 would be a dream on the high side.

Xram, how far forward and back will the eaton two speeds interchange chunks?

If you are concerned with accuracy when counting wheel turns as you turn the driveshaft. Turn the driveshaft untill you get 10 turns on the wheel, then take the number of times you turned the shaft and divide by 10. This will give you a closer answer, and if you use 100 turns then even closer, and so on....

About the wheel thing mentioned earlier on in the thread.
If you think that changing to 19.5" rims will make you go faster than keeping your 20" rims think again.
The tallest tire you can get in 19.5" is 8X19.5, and they are about 31" to 33" tall.
The 8.25X20 tire is close to 38" tall.
The taller the tire the faster you go.
If you want modern wheels the dirrect replacement for a 8.25X20 tire is a 9R22.5.
However, I have yet to find a 22.5" rim with our small 10 lug pattern.

I am currently running 8R19.5 and am thinking about modifying some wheels so I can run the 9R22.5.

Grigg

4.3in84toy:

According to the parts books, the carrier was the same for all '39-'46 (mech. shift) 2-spd. rears. According to the specs, those years all had the same ratios, too. I think that's through '46 first design.

From what I can tell, all 2-spd chunks from '39-'55.1 would bolt to any '39-'55.1 2-spd housing. As far as direct interchange goes, I can't say for sure, since axles (splines) and internal components changed over the years.

I do not think the 2 speed in my truck is original. I do not know what it came out of. It is mechanical shift. I know my high is a just less than 6.25 and my low is just over 8.

A 5.64 high would really help.

That's close to what was used in the '46 models, before they switched to the vacuum shift units. Ratios would have been 6.03 and 7.99 (I'm assuming the ratios changed from the earlier ones for the '46.1 models--the parts books don't distinguish between '46 first and second design). Serial number prefixes for the '46 second design 2-spd would be PJ or PK. '46 first design serials should start with an O.