I looked up composite road wheels. The set that I found from Lancer was 422 pounds lighter than aluminum. I don't know how much lighter the Soucy track is, but the Economist pointed out that M113s using a Soucy track were getting about 4 MPG, and the Soucy can use a lighter suspension design as well.

The key question left is how much engine you need. These lighter parts could mean using a smaller engine for the same speed or the same size engine with a greater top speed.

As for optimizing a vehicle for efficiency verses power, there are some companies that are working on camless valvetrains. Basically, they can shut themselves off instead of idle, purr along quite efficiently, and then put down a lot of power, because they have no limitations from the grind of the cam lobes and no parasitic loss from having to turn two or four cams, especially in DOHC configuration.

The downsides are that they are loud, can't go over 5,000 RPM, are expensive and require a 48V electrical system.

One last point on the rubber tracks... There was a newspaper article around a few years ago stating that TARDEC was looking at putting fiber optics in an Abrams, along with the gun from FCS and a rubber track. They anticipated being able to save something like 3,000 pounds in copper wire alone.

Perhaps we may reach a point where things start to get lighter instead of heavier.