For the long haul, constant speed is pretty frugal, so shouldn't make you need too big an engine.
Especially if you can be satisfied with some sensible speed; at cruising speeds aerodynamic resistance becomes the main drag (heh.) on fuel consumption, and that rises with the square (or cube?) of the rise in speed.
But staying under, say ninety MPH and with at least a reasonably aerodynamic body (as modern low-fuel and hybrid cars tend to have), I'd guess something like a 1.2 - 1.5 l three- or four-cylinder petrol producing 100-130 HP would be more than enough to both propel you down the freeway and re-charge the battery at the same time.
Perhaps even overkill; might be you could get away with little more than half that. The BMW i8 has a 1.5-l I3 turbo with a comparatively humongous 230 HP, but then that's a sleek mid-engined supercar wannabe thingy.
But staying under, say ninety MPH and with at least a reasonably aerodynamic body (as modern low-fuel and hybrid cars tend to have), I'd guess something like a 1.2 - 1.5 l three- or four-cylinder petrol producing 100-130 HP would be more than enough to both propel you down the freeway and re-charge the battery at the same time.
Perhaps even overkill; might be you could get away with little more than half that. The BMW i8 has a 1.5-l I3 turbo with a comparatively humongous 230 HP, but then that's a sleek mid-engined supercar wannabe thingy.