I've got a coffee cup stirling on my desk at work...pretty neat, and I know that you can buy stirling cycle fans to set atop your wood fire to move air around, kerosene powered fans in India, and that NASA messed around with them (used to have a pop science mag with a stirling powered Mustang in it...
But, as per Jimmy Olson's thread in General and O/T, 100MPG by definition requires that a gallon, 44.8hp hr is your entire energy budget for the distance.
Do it in an hour, and you need a car that can push through the air at 100MPH, and only use 44.8hp to do it...AT 100% efficiency.Do it in an hour and a half (65MPH), and you've got 30hp requirement.
NASA's best efforts to date are 38.5% efficient, and I'm seriously doubtful that the peltier gimmick can improve on that by more than 10% relative (3.85 thermal), but I'll be generous and let him have 45% efficiency.
Leaves him with a requirement that whatever vehicle it's installed in can ONLY use 13.5hp engine hp at 65MPH, on a dead flat road...need to accelerate and stop and climb hills, then the power budget stead state has to drop below 10hp.
So you see, the problem isn't a 100MPG ENGINE, it's a CAR that has to do it.