[swordtart]

If you compare power factors (i.e. hauling capacity) vs mpg the larger gas engines are more efficient.

For real fuel economy however you need an oversized vehicle. The largest gas engine is capable of hauling 90,000lb for 5 miles on a gallon of gas. With a even a well armed and armoured rig 40,000lb of that could be payload.

You would need 100 of the smallest gas engines to haul that (and far less would be payload). They would also burn 6.25 gallons of gas for every mile between them.