Lots of competing ideas here, but the OP is pretty correct in his original assertion. Just because an engine is "able" to get massive amounts of HP, does not necessarily mean it gets really poor mileage. Case in point is the Hellcat or Corvette. 600-700hp, but still able to get 22 mpg. Yes, they could do better with smaller, more optimized engines and net 29 mpg, but you can't always make things apples to apples. Life is made up of compromises, but fortunately for a diesel, the difference in mileage numbers between a 200hp engine pushing a full sized truck down the road and a 400hp engine is negligible. The real difference is how the driver drives the vehicle. If you use more hp, you are going to get poorer mileage, it's physics pure and simple. As for massive mpg gains to be made in the future from an internal combustion engine, not likely or it would have happened long ago. That thing called physics once again gets in the way and a certain amount of heat (btu's) is going to be wasted through the radiator or the tailpipe no matter what.
Adam
