The Stovebolt Site Forums The Shop Area Engine Shop 2 speed axle?

I've never had a two-speed axle, so I'm guessing that the lower ratio is extra low and the upper ration is standard?

I would like to lower my RPM at freeway speed, and have access to a two speed axle on the cheap.

The axle is in a larger truck, so not sure how well it would fit.

You realy won't gain anything swapping to the 2 speed. Those were generally used in 2 ton an larger, I have a few 1 1/2 tons with them, and are geared much lower than a 3/4 or 1 ton, even in the "high" gear. We're talking in the 6.50 range for high, usually in the 7s or 8s for low.
If you're loking for a more highway freindly final gear ration, there's 4 options, not counting bigger tires. 1, swap out the 3rd member for the late '60s/early '70s 4.10 ratio version; 2, swa the entire differential for a '70s or later diff with better gears; 3, overdrive transmission; or 4, add a Ranger Overdrive ($$$) or Gear Venders Overdrive (even more $$$$$$) to the existing trans. The easiest and cheapest would be the 3rd member or rear end swap. If you had a 1/2 ton, a 3 speed w/Borg-Warner overdrive or T-5 transmission would the way to go, but being a heavier truck, you need something with more stength. Choices are limited, and none of the HD overdrive offerings are a direct bolt on with the stock bellhousing. They can be adapted, though.

yeah, my '59 f700 2sp is great for pulling trees out of the river in lo, but highway in hi is 'comfy' only at 55mph or slower

Will any of the original overdrives bolt to the back of the 4 speed transmission in my 49 3/4T (torque-tube to center bearing), or do they only fit the 3 speeds?

The Borg Warner overdrives only fit a 3 speed. I am in the process of installing a 3 speed with overdrive that must have come out of a pickup because it has a U-joint instead of a slip joint. Its going into a 50 GMC 3/4 ton with a 302 and the original 4.56 gears. I will have a short drive shaft with a center bearing made and use the original rear drive shaft. T89 Warner overdrives were available as a heavy duty option but they are very rare. Most including mine have the Chev front section with the Warner OD behind it. If you require the deep first gear of a 4 speed, you might look for a 5 speed with 5th over. They are hard to find too.

The "Columbia" overdrive rear axle was available on Ford, Lincoln etc. pre-war. It replaced the right side of the differential with a new housing and an overdrive sun and planet gear set operated by a vacuum piston. It's called a 2-speed, but this isn't accurate because the differential operates normally (no extra parts engaged) unless the overdrive set is selected (which can be in any gear, giving 6 with a 3-speed etc.).
The ratios were standard and overdrive, but I can't identify the exact ratio.