One of the things I like about my LS1 is that it makes great power but still gets good economy on a trip.
We've had to comply with emissions laws down here for years with our Cobras. If you build one it has to be registered as a new car and concequently has to meet all of the design rules set down for new cars. We have to source a motor that meets the current emissions lars (Euro#3 from memory) and run cats etc. Economy isn't a requirement yet but these modern motors get good mileage just because of their eficiency and design.
With the T56 6 speed pox and it's 0.5 OD 6th gear the car is loping along at 1500RPM at freeway speeds. On a trip I can expect 10L/100K which is about 23.5 MPG
Some of the new Diesel engines ar becoming quite stout performers. I've got a 2 litre turbo diesel in my VW that easily out performs it's 2 Litre Petrol stablemate. It still returns 5.2L/100 (45MPG) on a trip and 6.2L/100 (38MPG) driving around the city. It's fairly clea and only puffs a bit of smoke when you put your foot down. It's quiet and the only time you know it's a diesel is at idle or low speeds. Once you are up and runing it sounds like any other new car.
Audi's current Lemans dominating sports car is a V12 turbo Diesel. Check out the Audi R10:
Audi R10 Maybe this will be the future of Green perfomance engines?
Cheers