My conclusion is that an underpowered modern vehicle will overwork the engine and bring the MPG's down while an engine that is adequate to a bit more than that, when driven properly, will get the most MPG's from the vehicle in stock form. The 1970's low compression smog motors were the worst offenders by far in terms of ******* gas and not delivering on power. What most impresses me about modern sports cars is that light weight combines with powerful engines, that if carefully driven, will come close to delivering the same MPG's as popular economy cars like the Honda Accord.
Your mileage may vary...LOL!
Rick