[shovel]

You could gut a transfer case, so that the chain and front output shaft aren't there... if you're ambitious you could cut the whole bottom of the case off and fabricate a cover, then make up the difference in gear ratios by using a numerically low rear axle ratio. You could "clock" the transfer case up so lubricant still makes its way to the gears..

Transfer cases common to Chevy and Jeep vehicles like NP231, etc are 2.72:1 in low range.

An important consideration is that transfer cases aren't intended to be shifted between ranges while driving, there is no clutch between them and the transmission so even when you are in neutral on a manual transmission, there's the (gear-enhanced) inertia of the transmission's spinning parts to fight against with no synchromesh.

Old Monteros have 1.91:1 low range, which might be more fit for this sort of idea. Several Borg-Warner transfer cases (often found in Fords) are 2.48:1 . Nissan 4x4's are generally 2.59:1 and I think there was one 2.02:1 but I don't recall which model. NP203 and NP205 transfer cases, on some Chevy trucks are 2.01:1 and 1.96:1 respectively. There are several aftermarket transfer cases (Atlas,Marlin for example) but they're $$$ and also several aftermarket add-on overdrive units intended for highway MPG on old, non-overdrive vehicles - also $$$

In the off-road world, some guys have frankensteined up multiple transmissions, or multiple transfer cases stacked end to end to get a broader spread of ratios.. but anything like that is adding complexity and weight and rotating mass and friction... none of those sound like a good idea for efficiency.

I think the basic concept starts off with merit, but there's probably no cost-effective measure for simply using a transfer case in this way. My opinion is the juice won't be worth the squeeze, and there'd be a lot of squeeze just to get there.

Early Colt/Champ models (1980ish) had a 2 range transaxle, so you had 4 forward gears and 1 reverse gear, but a "power" and "econ" stick next to that which allowed the driver to select something like 10% different ratios, remember this was on a carbureted 1.5 or 1.6L engine so there wasn't a ton of engine torque available to offer a vast difference between the high and lower ratio. I left mine in "power" mode all the time unless I was exclusively going to be on the highway.

These user-adjustable controls of course are vanishing from all cars because the engineering attitude is "make the most features available to the most users" - i.e. there are actually (MANY) 4x4 owners who don't know what that extra lever is for, and while the best option for mankind's viability on this planet would be to neuter those people so they don't pollute the future with their inobservant, oblivious DNA, that doesn't sell cars so the manufacturers replaced the lever with a button... and then with full-time AWD or one-size-fits-all software voodoo that is neither as fuel efficient nor rugged as conventional 4x4 with manual hubs on the older generations, but lets ignorant operators reap the benefits without undertaking any effort to understand the 2-ton tool they're piloting. Idiot proofing is nothing more than a race to bigger idiots, but that's another topic entirely. The point here as it relates to this thread is that the economy-minded underdrive DID exist once on OEM cars, and then became that "OD OFF" switch on auto levers, and is approaching extinction today. If you want to put one in a modern car, it'll be some Marco Polo action for sure!