Gas engines are only 20% efficient meaning that actually only 4 gallons from a 20 gallon tank are used in actual combustion. The remaining 16 gallons is wasted.

More than 20% of the gas is combusted...it's just that only 20% of it actually turns into power.

Isn't the Renesis engine in the RX-8 supposed to be more efficient at turning fuel into power? I can't remember exactly but isn't it suppose to make something like 250 hp being N/A and get like 25 miles to the gallon? I could handle that over my 100 hp gas-chomping 12A. The rotary of the future sounds pretty nice.

I believe you're referring to an actual automobile's efficiency in which roughly 20% of the work it does is used in making the car move from point A to B. The rest is used in powering things like AC, PS, Radio, etc.