what is an under drive transmission i have heard it mentioned in more than one thread now and don't know what it is
We call them Browning Boxes It will be like an AUX Transmission, we call them Browning Boxes where I live. I guy I used to work for had an Old International Bob-Truck with Combine tires on the back. The Tall Tires had the gear ratio all screwed up so there was a Browning Box mounted in the driveline between the Rear of the Transmission and Rear End. I think it was a 3 SPD: Underdrive, Direct and Overdrive. We left it in Underdrive, this got the gear ratio back down where it was driveable. It was like the truck left the factory with Combine Tires on it. Underdrive is like overdrive but instead of going faster you go slower. These were popular along time ago when the engines were a lot smaller than they are now. That's how they pulled those big loads with those 200 and something horse engines. They may still be popular in the mountains:beatsme I live in the flatlands and small hills. An 8LL and a 400 H.P. Engine is better than most have here.
When we got our first tandem dump, it had two transmissions a five speed main and a three speed auxiliary. Middle gear on the auxiliary was direct, top speed was an overdrive and of course bottom was an underdrive - output shaft rotated slower than input. The shifter knob was marked U, D, and O. Back in the sixties a Roadranger was a complicated new thing. After all, who in the world would you ever get to fix it!
Underdrive was in most auxilary transmissions, but they started to put them in Roadrangers in the early days. They shifted DOWN rather than up, so if you weren't familiar or at least aware of it, you could lose a tooth on the steering wheel. They were in the grossly underpowered trucks with 6-71 detroits and the likes, to be able to grab a half of a gear down without moving the stick. No matter how long you were driving, how old, how many miles you drove, how good you thought you were at it, at one time or another you were going to hit that button thinking you're grabbing a cog in the proper progression, and end up hitting your forehead on the windshield.
My Oshkosh has underdrive. Two transmissions. First the auxilary has under, direct and over drive. The main is a 4 speed with 1 reverse. Gives 12 gears foreword and 3 in reverse. She gets down the road at a breakneck speed of 45 miles per hour in 4th with over drive. For plowing snow with Vee plow and wing usually use direct and 2nd or 3rd.