I'm a six cylinder guy plain and simple. I4s sound too buzzy as do small displacement flatplane V8s, high displacement crossplane V8s are a bit deep for my tastes.

If I built something like this I'd probably take a Nissan VQ30DE out of a wrecked maxima ($400ish with everything), drop it in a $500 craigslist 80s Nissan 300ZX (very aerodynamic, not quite as aerodynamic as a 93+ RX7 or a 240SX but much less demand), give it the same (if not even more drastic) 500+lb weight reduction effort I put into my own car, a few electronic tricks here and there, and I figure I'd have around 220hp, 2300lb- (loaded with driver) car. Couple that with my driving techniques which are already proven to deliver great fuel economy and I could probably hit 50+ mpg at least on the highway. Maybe if I could have it tuned to lean it out during cruising conditions I could make significantly more.

I don't know enough about diesel engines to be comfortable working on them and I'm sure maintenance would soon claim any money saved on fuel. Not to mention diesel fuel pressure scares me on it's own. I have no idea what kind of 4 banger I'd go for for fuel economy. Bike engines are the smallest and at over 13:1 compression you'd think a Yamaha R6 engine would be a good choice given a CVT transmission but in the bike it comes from it gets 35-38mpg highway (according to my friend who owns one). The Corvette Z06 has since it's beginning shown how possible it is to get good fuel economy from a BIG engine in a fairly heavy coupe (yeah it's not quite a 3500lb porker but it's no sub-2400) so at least for me it's impossible to guess fuel economy based on engine size.