Has anyone here had any experience with a Gear Vendors overdrive unit? I can read testimonials on their site, but I would like some input from "real" users. Thanks  

I have a gear vendors unit on my 65 F250.  I have a T18a manual, and there is a unit that bolts directly to the back of the tranny.  I live near
Unitrax shop, so I had them do the work.  The truck has 4.10 gears in the rear and 225/70R19.5 wheels.  I am very pleased with my results.

For the good side:  The overdrive realy brings down the RPMs for freeway driving.  Before the truck never saw anything over 60 mph!  Now I can cruise very comfortably at 65 all day.  I can get at best about 15 mpg on the highway, better than the 11 I used to get!  The unit has two switches:  A master on/off switch and a pull switch mounted on the gear shift lever.  It is easy and comfortable to use, and I can split gears with a third overdrive as well as fourth overdrive.  I gear split only on steep grades climbing the Sierras with all my camping stuff.  The whole package is very foolproof and easy to use.  I have had my package for 5 years and have not had any problems at all.

For the "bad" side:  I only have a couple of minor complaints, but nothing to make me regret in any way my purchase.  The gear vendors unit does have a computer control that ties into the vehicle's electric system.  The computer will not allow the overdrive to engage at speeds lower than 25 mph.  I am not able to have a second gear overdrive to fill that gap as a result.  This speed regulation protects the clutching mechanism inside the overdrive unit.  If I had taller (numerically lower) rear end gears, I would not have this issue.  My only other complaint is the pull switch on the shift lever.  At freeway speeds, the switch rattles very loudly.  Ona bumpy section of freeway, the switch can rattle just enough to momentarily turn off the overdrive.  Witha loud thunk, the truck will downshift then moments later upshift again.  Many times I drive with my right hand resting on the gearshift, holding the switch up and keeping it from rattling.  I may try a different kind of switch some day, if I can get my lazy butt around to it.

Overall I am very pleased with the unit.  I did not research much into the overdrives by US Gear.  I think they may have a manual shift option for their overdrives.  Good luck with your search.