[TurnOne]

2004/5 Mazdaspeed Miata 6 speed vs NA 6 speed show 2 mpg (combined, city and highway) advantage for the NA model. 9.5 compression ratio compared to the NA's 10.0.

Only way I can see a normal factory turbo car achieving better highway mpg is to use a smaller displacement engine. Smaller motor means less friction, right? Assuming both are putting out the same HP at (say 60 mph for example) to make the car overcome the friction of going down the road.

Adding a turbo would only add power, and any theoretical gains would be wiped out as soon as you boost one time more than you expected. Everyone mashes the accelerator to the floor sometimes right? I have a bunch of vehicles. Some turbo, some diesel turbo, some NA. Most with manuals, one with TC and one with CVT. Everyone of them gets the full throttle treatment sometimes.