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You're looking at two totally different scales.
That 30% efficiency includes drvetrain losses, road friction losses, losses due to heat, losses due to accessories, etcetera.
The engine itself is nearly 100% efficient at converting fuel into power (except the fuel that is used to cool the engine and light the cats). But only 30% of that power goes towards pushing the car around. If you increase the efficiency of the engine itself by one percent and ignore all the loses caused by other stuff, then you're only going to gain a fraction of percent gain in efficiency overall.
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