I guess it all depends on where you are looking at it from.
I get stuck on looking at it from the project standpoint, so I figure as much compression as whatever engine I'm building will stand on pump swill (the cheap stuff, I like to drive my stuff alot) and keep the crevice volume small with a reasonable cam, and away I go.
Of course my preference for lightweight vehicles doesn't hurt.
Radials are a help with economy (I prefer to think of it as range).
I was recently reading about a vehicle that was competing in one of the X prize competitions(this one was for economy in vehicles that were street legal, met the 2011 emissions standards, and were producible quickly, with today's tech) about a Chevy Metro that had been converted to turbo diesel.
It was an engine swap predominately , with a few aero mods to help at freeway speeds.
It was getting better than 50 mpg on the "in town" part of the test track, with just over 70 on the "freeway" portion.
Best part was that the group responsible for it was going to offer kits and consultation to help duplicate it.
Sad part of it is the "was".
For less than half a Prius, you could build yourself a miserly ride.
Or take a road trip, and not stop for fuel, until you want to.
