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According to Fueleconomy.gov website here are the stats that seem correct:
Petrol vehicles are 12% to 30% efficient,
Engine losses: 68% to 72% (thermal losses 58% to 62%),
Wind resistance: 8% to 12%,
Rolling resistance: 4% to 7%,
Braking: 4% to 7%,
So that would make engines overall 28% to 32% thermally efficient.
Combustion losses: 3%,
Friction losses: 3%
Discounting pumping efficiencies, we can improve engines theoretically by 64% to 68% (adding thermal, combustion and friction losses).
Compare that to 8% to 12% wind resistance losses.
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