[Big Dave]

I can relate to Phil’s problem.

Gearing has helped me. It is to the point that I need to re-gear again to fully reap the benefits of my aerodynamic efforts.

Both gas engines and diesels have really good reasons why gearing improves MPG.
Both engines have a frictional drag involved in just the mechanical running of the engine and pumping the fluids around in the engine. It is not an inconsiderable amount of friction. According to curves I obtained, dropping my gear ratio from 3.73 to 3.08 reduced my engine frictional HP by 12 HP at 70 MPH. If I could drop my ratio another 30% I will get another 8 HP frictional reduction.

In addition gas engines suffer another penalty for excessively high(numerically) gearing. Gas engines modulate power output by throttling. Gas engines are married to a very narrow mixture range, so less power means less air flow which means you close the throttle and make the engine less thermodynamically efficient. A gas engine is most efficient at wide-open throttle. That is what makes hybrids efficient. They use the batteries to help a small engine cope with varying road loads and that small engine operates WOT a lot of the time. A true series hybrid where the engine only charges the battery and never sees road load could use a single engine speed: WOT.

If he still has the OEM gearing I do suspect Phil has hit a wall. As stated, there is no aftermarket support for Toyota axles. But Toyota stuff often resembles GM stuff. My advice would be to get an old GM 10-bolt axle. If necessary, narrow it to fit his truck. Narrowing axles is not for the DIY and will not be without some cost ($300-750), but a GM 10 bolt will allow Phil a wide range of axle ratios all the way down to 2.47.

I cannot reduce my axle ratio unless I degrade the carrying/towing capability of the truck. So I either have to substitute a T-56 transmission or put on a Gear Vendor or US Gear overdrive unit. The GV and USG will reduce my engine RPM by 20%. Both are fairly easy if pricey add-ons – about $3500 installed. Problem is that I wind up pushing not one but two overdrive meshes. The T-56 (there is a 650 ft-lb version made for Vipers) gives me a 31% lower engine RPM and I only have to push one overdrive gear mesh. Every gar mesh robs some frictional HP. In these big trannies the gear frictional HP is so large that these manual transmissions have to have external transmission coolers. The optimal drive train for my truck would be a seven-speed Spicer with a straight-through top gear and a 1.56:1 rear axle ratio, but no such axle exists.