Higher power would mean less throttle required to match the acceleration graphs on the test equipment ,, should mean higher mileage ... at least as far as a hemi 300c is concerned as the car is substantially unchanged... I don’t think the test has changed much ,, you should be able to make valid comparisons between current and prior years ... http://www.nhtsa.gov/fuel-economy
Lower powered cars operating at closer to max power are more efficient than high powered cars operating at lower power levels. Stated another way...
a 100 horsepower engine operating at 100% output is more efficient than a 200 horsepower engine operating at 50% output. Both are generating 100 horsepower but the more powerful engine consumes more fuel.
The function of a throttle is to impede the flow of air into the engine. This has two effects: (1) reducing the amount of air per stroke; (2) adding drag by forcing the engine to work pumping vacuum. The latter effect is in many cases the more significant one, but the work spent pumping vacuum, which can be significant at low throttle, is essentially wasted. Such waste is much greater in a large engine at low throttle than in a small engine at wide-open throttle.
“Higher power would mean less throttle required to match the acceleration graphs on the test equipment ,, should mean higher mileage ... “
That’s just silly.
You can’t get free power, it all has to come from the fuel.