We're talking about an "engine that gets 80MPG".
Such a statement is like saying that the Hummer H2 must be a fast vehicle because it has a 300+ HP V8 in it.
An engine by itself does nothing. It gets 0MPG and goes nowhere.
When installed in a vehicle so as to be capable of performing useful work, engines are part of a system. The characteristics of the other components in that system, such as rolling resistance, transmission losses, gearing, aerodynamic drag, and vehicle weight, just to name a few, factor heavily into the systems' performance in terms of fuel economy and acceleration.
I know of several "80MPG engines". They just happen to be on motorcycles, as opposed to a 3000lb sedan.
By the way, hybrids are fuel injected and don't use carburetors. I was making reference to this (although this is a 200MPG carburetor):
http://www.snopes.com/autos/business/carburetor.asp It seems to me that the impetus for increased fuel economy due to CAFE would be reason enough for any automaker to pull the wraps off of this "80MPG engine", if it indeed exists.
You also recall the sudden disappearance of the single-turbine engine? Chrysler made one that worked in the mid-1960's--and the accountants KILLED the damn thing.
Not only extremely efficient--but almost maintenance-proof, and only 3 moving parts. Hardly the way to make money in the aftermarket.