i have no idea what this maens!!!!!!!!!!!!!!!!
Oh are you going to have fun!
I know about enough to spell it differently every time. There are work measurement conversions (HP to KwHr), weight measurements (g/kwhr to lbs/hp) fuel mass calculations (regular, premium, E-10) and a whole lot more. Rather than reading somewhere else and then regurgitating that here, I suggest you do a search and plan on sittling down for a few hours to get familiar with the terms and how they convert. Start on Wiki[pedia and look up BSFC (Brake Specific Fuel Consumption) To work out the best engine, I think you will have to know the weight and gearing of your car, some nubers for aero and rolling resistance etc. The previously mentioned calculator and Driven's numbers are good starting points. Beware of " paralysis by analysis".
Or you can just build something and see where it goes.... Unless you get something like my gas guzzling rotary, you will probably get between 25 and 35 MPG at 70 MPH. I only get 20 MPG with my rotary.
Oh I just saw what horizonjob wrote. As he said, to maximize MPG assuming the same vehicle, smaller engines working harder is better than bigger engines working less hard. But he and I disagree on the math for your HP. His 120HP assumes a 1200 lb car. very few of us get that. I'm in at 1400# not counting driver/ passenger/ load. And that makes matters worse for fuel efficiency
So we basically killed your 40MPG goal, now we're rapidly killing your 200HP/ton goal. What are friends for?