Big block 540 - Decent gas mileage

Those seem to be mutually exclusive statements to me.

While I agree with tcodi on the mass and aerodynamics to a certain extent, I can't buy the 30 mpg big block Civic. Volumetric efficiency also plays a factor. Assuming optimal Air Fuel ratio is about 14-1, air to gas, if you bring more air in to fill the cylinders you are going to bring in more gas too. Modern fuel injection and computer control has gone a long way to improving mileage, but in most cases a bigger motor is gonna use more fuel. The exception would be if you underpower a heavy vehicle.

Back to your question, could you get 20 mpg on the highway. I don't think so. Could you get 12-14? Possibly if you keep your foot out of it, but where's the fun in that? Perhaps a better choice is to build one of the new generation EFI motors that will run hard and still pull down 20+ mpg.

Pat