Underdrive/overdrive units ???
 

:confused: I keep seeing threads about low performance, low mileage figures-both gas & diesels. Also low gear ratios vs higher gears for mileage. BUT- no one seems to have ever tried an OVERDRIVE/UNDERDRIVE unit. It can be used with 4X4's just not in 4x4 mode. In two wheel mode (highway) it would seem in overdrive mode (non-towing) mileage would increase exponentially considering the torque of the diesels. Use underdrive for towing only......They units can be moved from vehicle to vehicle so cost can be spread out over several vehicles/years......So how come no one has tried it???? or have they????:confused: :confused:

If you mean something like the GearVendors drive used in trucks, RV's etc, I've used them in the automotive application, but I don't think the small ratio change would work well in a boat. We have used turbo 400's in our race boats for many years...

:eek: I think you misundertand me....I am talking about towing & highway usage of over/underdrive units to get low gears for towing & high gears for fuel mileage while maintaining the advantage of large displacement low geared vehicles...ie: boat ramps, mountains, etc. with 11,000# loads.
So, we could get a truck with a 3.55:1 ratio, add an over/underdrive & get 5.88:1 for towing and say 3.55 or 2.73 for cruising with no load.....??? Still, has anyone done this???:confused:

I have one on my Street truck and highly recommend it; the unit has been absolutely trouble free .also mounting the shifting switch on the floor or the gear lever..

Best Regards
:)

Blown Formula,
OK I was on the right track, it was for your truck. The Gear Vendor over/underdrive I used for years for towing was an outstanding gear splitter. It will give you an overdrive in every gear. Its a little tricky to get all six or eight speeds smoothly but with a little practice you'll be a pro.
I think the Gear Vendor was out of San Diego or Orange County in So Cal. Your truck or RV aftermarket people will know...