[Big Dave]

On the face of it I agree with stillsearching. I would love to substitute a 1.6:1 rear axle into my F-350 and a seven-speed Spicer with a straight-through top gear. One gear mesh between flywheel and the tire. Optimal, but alas! Unavailable.

Ever try to find axles under about 2.73:1? My 3.08 is practically a one-of-a-kind. Even for dinky little GM 10 bolts, 2.56 is the lowest I ever heard of.

In my younger days you could get custom-hobbed hypoid gears. Maybe you can today, but I don’t know where.

Part of the problem is that the differential typically nests inside the ring gear. If you reduce the ratio the ring gear diameter has to reduce and then where do you put the diff? You want the diff as close to the vehicle center line as possible to avoid having unequal length drive shafts. (Unequal drive half-shaft lengths is what made the torque steer on old Saab 900 Turbos so wicked) You can’t reduce pinion size, that would allow engine torque to shear off pinion teeth.

FWIW slowing down the engine speed at a given road speed improves MPG because it reduces the engine friction loss. This is somewhat of a misnomer. The mechanical friction is almost negligible. Where the loss comes in is pumping air and exhaust gas through the engine. The torque needed to pump air goes up with the square of volume moved, hence the power goes up with the cube of volume moved. All air paths have a low range where the viscous resistance is greater than the compression power requirements. For My International 444, that is about 1800 RPM. From there the pumping loss skyrockets. At 3300 RPM power actually drops off as the engine consumes a greater and greater amount of power just pumping air.

So the slower the engine (a positive-displacement air pump) runs for a given road speed it pumps significantly less air and requires significantly less engine power to do so.

The theory works beautifully for me, even to the point of overcoming the inefficiency of having two gear meshes between the flywheel and tire.