OK, you want to run on pump gas. Forget about fuel mileage, it that's important to you, build a Geo Metro 1 litre. What is the vehicle? Will you have to deal with emissions? What rear gear? What transmission? What torque converter? Or will it be manual trans?. How much will it weigh? Will you tow with it? Will you off-road with it? The more detailed you can be, the better information you will get.