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In the old Nissan rear wheel drive 4 and 5 speeds, 4th "gear" was direct drive (input shaft and main shaft coupled together). It would work without any other gears or even the counter shaft or the counter shaft drive or driven gear. Basically a shaft from the clutch to the prop shaft.
If the trans axles are the same with one gear that is direct drive then it would be the most efficient in that gear.
Sounds like 90% is fairly close when you average the variables together. Another way to figure it out is to dyno the engine then the whole car. In either case the load will change the efficiency. Dynoing the whole car will give you a pretty good idea of losses through the whole power train. Probably in the 15-20% range for RWD cars. It would be lower if the tranny used ATF like the 60s Mopars.
Drag racers like the old power glide because it lost the least amount of power to the rear wheels.
At highway speeds with low engine power 90% is probably fairly close in a trans axle.
regards
Mech
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