I've tried to get as much bicycle efficeiency and as much of the experience of cycling combined into my builds, but always within PRACTICAL limits-
But that's why I have 700C and 27 x 1 1/4 tires, and use as much alloy parts as I can- from years of road cycling. The top end just goes up a bit as you strive to get more practical efficiency- I'm sort of an enviro-tinkerer- but I think there's a real practical top end about 35- and even that's too wide open to seek continually- My 66 build is really comfortable with itself between 25 and 30- so that's usually where I get on open stretches.
Who has the greatest MPG is hard- because, well IT'S A BICYCLE, and you can just pedal the thing across the country with a gallon of gas in the tank probably. I probably am up in the higher echelon there, because I just do a series of "Burns" quite a bit- I love to coast, and I hate to waste brake pads and stress out braking, so I get up to speed and as far as I think I can coast from any anticipated stop, I kill the motor and coast away until I'm at that point at about normal cycling roll/ speed.
And with the semi-skinny tires I'm really quite comforatble with, cause they're not really like sew-ups or even 23mm clinchers- the bikes roll a half mile in calm wind once I'm up to speed. They're really sort of like a classic 10 speed in ridability, except I use a modern lightweight cruiser frame for clearance, and only one build has a mult-speed freewheel in the back.
So anyway- still enjoying myself and haven't had any real problems yet- 3 1/2 years on.