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Weight has a linear effect on energy requirement. Frontal area (drag coefficient) has a cubic effect on energy requirement. Agree that 500lbs of weight isn't explaining the difference in fuel economy.
The 5.3 V8 is a less efficient machine, period. Pushrod, NA, no DI, heavier/more friction rotating assembly, etc. The fuel economy illustrates this. The example given is a pretty good one with similar output and similar size/weight vehicles.
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