The Gun engine works by harnessing detonation (fuel explosions) instead of the normal rapid burning of fuel (deflagaration) that our current engines use. Detonation is a LOT more energetic than deflagaration. So energetic, in fact, that it destroys car engines (usually by blowing apart the pistons inside the engine) when it occurs. The fact that the guy's prototype, while 92% efficient, destroyed its own crankshaft while running shows you how powerful detonation really is. The fact that his pistons haven't blown up certainly shows some promise, though.
In any event, color me skeptical. I have no problem believing that there's a much better way to do things than the current 30-35% efficient gasoline engines in all our cars. (Which happens, ironically, because we run our engines far too cool. The temperature differential puts the upper bound on the efficiency of an internal combustion engine.) But 220 MPG seems like fantasy. Though, I suppose, if my Civic can get 45 MPG now, and this guy's claims of quadruple efficiency are actually true, 150 MPG might be possible. Well, this will just have to be one place where I'll just have to hope my skepticism is unfounded...