my stock 350 in my chevelle gets 16mpg and thats me milking it. 3000lb car, so i think u need to redo the calculations.
There is more to making mileage work for you than just displacement and weight. Granted, it will take always less fuel to feed 200 cubic inches than it will to feed 500 cubic inches, but it is entirely possible to increase the efficiency of a bigger engine while minimizing it in a smaller engine and you end up with equal fuel consumption.
I've got 100 cubic inches and 1000 pounds over your Chevelle, yet I can match your mileage easily because I am also producing 150-200 pound feet more torque than your 350, which means I have to use very little throttle angle to get my truck moving and keep it there. It also comes in right off idle and steadily increases up to my cruise rpm. So while I'm running down the highway at 65 mph, my engine is operating at its peak efficiency range.
By the way, torque values directly correlate to volumetric efficiency. The the more torque you design your engine to produce, the more efficiently you use the fuel. This is why the OEMs have to compromise; the build cars for everyone from joey speed racer to grandma gerttie and jim bob trucker. They have to have a design that is compromised to suite all of them at altitudes ranging from sea level to mountain tops. As hot rodders, we can build engines that suit specific applications so we can maximize their output to suit those purposes, whether they are higher rpm horsepower or low down towing grunt and can do so specific to the area we live in.
