Gear Vendors Over/Underdrives are the sheet. Most people think they're spending the money simply to get an "overdrive" option...they're not, they're getting a lot more. The O/U's are "gear splitters" in essence, so while you add 28% or so to your final drive ratio, you also split the difference between 1st-2nd and 2nd-3rd. Comes in real handy when you have bad ratio combo to begin with--you alleviate all the "down-rpm" between normal shifts if you know what I mean. They make stand-alone units that work with torque-tubes, too. There's typically no strain on the tranny, actually less in 3rd-over's case. Still, having a full-synchro unit to begin with is helpful. I wouldn't run a GV on a real early manual trans myself.
