[Big Dave]

Back on topic.

Gearing changes allow you to fully realize the fruits of aero and weight improvements.

Aero & weight reduce the engine torque required at a given road speed. Changing gearing reduces the air flow through the engine (in itself a major cause of engine friction) and in a gas engine that means a lower fuel burn, all this at the same road speed. If you don't regear the vehicle yopu have to use a more closed throttle to acheive lower air flow and a throttle plate is nothing but an entropy generator.

You would think regaring would be more effective on a gasser than a diesel but the high compression of the diesel is another good reason to minimize air flow. Unfortunately, gearing is expensive. These arehigh-precision bits of machinery that require complex manufacturing tecniques which means high cost. Still, if you really think oil is going to $200/bbl, your choice may boil down to coughing up the cash for gearing or just not going.