1) @ 100-mpg & 60-mph
2) we're talking 0.60-gallons / hour
3) @ 6.138 pounds per gallon, that's 3.6828-pounds / hour
4) with Regular Unleaded, E10, that's 111,836-Btu / gallon
5) or 67,101.6 Btu / hour available
6) and @ 2,542.58-Btu / hour per horsepower = 26.391146 Bhp
7) and behind an overdrive driveline, 24.27985-horsepower road load.
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8) @ 3.6828-pounds / hour divided by 26.391146 Bhp
9) that leaves a BSFC of 0.1395 pounds per brake horsepower-hour.
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10) there's no I.C.-engine known with a BSFC that low. Only an electric motor.
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11) If we took 0.36 BSFC as a reasonable minimum ( gasoline direct injection turbo Atkinson cycle )
12) The brake horsepower would have to drop to 9.408
13) That is going to require a small frontal area, very low drag, and low mass.
14) And some other dave's comment about a diesel engine is spot-on, as this was Volkswagen's solution for their XL1.