Pretty much all the gains in modern cars are down to better fuel and ignition management. This has been demonstrated in instances where enthusiasts of older cars retrofit original engines with modern computer based injection and ignition. Smaller gains have been realised in friction and rotating mass.
Many modern engines are also running stratified charge, which was very difficult to implement on cars years ago before microprocessor control. Crucially however these methods don't give huge returns when driven about town, as when accelerating under load, fueling has to revert back to a stoichiometric ratio in order to get decent power. On long runs modern cars can give very good returns, as they can be made to run on almost fumes, as only a few hp is needed to overcome drag and rolling resistance.
With modern engines and construction methods, straightforward non-hybrid 100mpg cars should be everywhere, but they're not. No car really requires more than 1L of displacement, small cars can get by on half that.