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Has anyone added an auxiliary transmission, overdrive transmission or different rear end to gain a little more speed? I love driving my 1940 1-1/2 ton around but, as we all know, you max out at 45-50 mph. To me it is more of a safety issue. Every year, with more traffic, it seems there are more close calls with someone coming up on the back of the truck going 70-80 mph. I sure don't want to get carried away, as it still has stock brakes, but it would be nice to cruise at 55-60 mph. Is there a Spicer/Brown-Lipe that fits in the closed driveline? What type of overdrive was available? Thanks for any help or suggestion.

I wasn't aware that 1-1/2 tons had enclosed driveshafts. If you have an open driveshaft you can put an auxiliary behind the main transmission. I did this many years ago and am still driving the truck. The one to look for is a Spicer or Brown-Lipe 5831A. These were found in many tandem axle trucks, dump trucks are most common in my area. Or the next bigger transmission would be a 6231A. There is a fellow who advertises in the ATHS magazine who has at least one of these.

The "A" designation is important as those have the 1.24 underdrive and 0.86 overdrive the other have a 2.xx underdrive. Those are no use on the road; the under is just for crawling around on construction sites or farm yards. So you only split the main box gears twice rather than 3 times as with the A models. It makes a big difference on the road to have the extra gears.

You could also put a two-speed axle from a later truck but they were usually geared pretty low to accommodate 9.00 X20 tires.

5531 would also work, 6231 is very large, but will also work.

Ed

1940 had a 2-piece enclosed driveshaft all the way from the rear end to the transmission. I believe they changed to open driveline in 1941 on the larger trucks, IIRC.

I'm not aware of a box that specifically fits your enclosed driveline, but I have seen literature describing boxes that could be paired to enclosed drivelines, of course they required changes to the driveline enclosure. Finding one for your application will not be easy even if one existed 75 years ago.
I have a 5831C in my truck, all 3 ratios are road usable and split the 4 speed both up and down in every gear sequentially. See videos below for an example of how it drives. Overdrive is 0.85 so you can add about 17% to your top speed on level ground. As always, the rest of the truck especially brakes will need to be in great shape if you are going faster.

Check out this Wed Site http://6066gmcguy.com/ Look at the Spicer 5831 Aux Transmissions the ones you want are the ones with 0.73:1 overdrive. Any questions call me 1-262-392-2989

Marv.

I think the best bet for your '40, would be to change to an open driveline, lot of work but there are lots of unused rears and trans' around, might as well get an updated rear, two speed might get slightly better ratio(6.5:1?) so can fit in a Brownie while doing the work.

Is your wheel bolt pattern 5/10 on 7.25" or ? Could use 420, 465 4 speeds or 540 5 speed.

Good luck.

Ed

Are you still running a 216? Good luck with developing enough torque to pull an overdrive. Running downhill with a tailwind just isn't an option all the time- - - -eventually you've got to do some climbing.
Jerry