What's the best reguardless of cost, convience, speed? The only stipulation is it must be able to go at least 40mph, so no bicycles. The best I can see is bennelson's electric Kawisaki and the best production vehicle I've seen to date is http://www.myersmotors.com/ NMG!
Probably an aerodynamic gyro-train running a combined cycle powerplant: high-compression, turbocharged/turbocompounded diesel powering a steam turbine via waste heat. Vacuum sealed gyroscope on superfluid bearings...superconductors used throughout with ultra high efficiency motors/ultracapacitors for the traction drive system.
Gyro-Train
If you meant personal transportation, I'd suspect a faired motorcycle (a la Craig Vetter) would be your best bet.
A very efficient design would be a recumbent trike with a fairing and electric powered, solar charged would be very cool..cheap cost to operate and eco-freindly
biggest hurtles - light batteries.. the newest batteries would do this just fine however costs are very high.
Why no bicycles? if I stepped my bike up to 72v it should be able to do 45mph.
My comuticar is supposed to do 40mph with a good new set of batteries and with a few upgrades should do 40+, current is 35-37mph with snow tires, needs an alignment and has old abused batteries. My last Super Mileage Vehicle would do 45mph, got 245mpg, but wasn't road legel, I did build it for about $600 tho.
