[PaleMelanesian]

Take two cars of the same model, with only the engine different. Say a V6 and a 4-cylinder. Now apply the same aerodynamic mods to both cars. It will make the same aerodynamic improvement on both. The engine gives a different baseline mpg, but that is independent of the mod's effect. You'll see the same (or very similar) percent improvement in both cases, but it's a different absolute mpg gain because of the different baseline.