It's semantics, but in this case, semantics do matter. There's a real difference in result between taxing gasoline per gallon and taxing the ownership of a fuel-inefficient vehicle.

The latter is roughly equivalent to the US's CAFE standards and leads to gaming the system by manufacturers.

The former is much harder to game - either you use the gasoline (and paid the tax at purchase) or you don't. Doesn't matter (to the tax authority) what car you drive - only how much gasoline you burn.