It is true that this may be simplistic, but scaling down vehicles doesn't only mean scaling down weight. Some of the big SUVs are just way too large no matter how you look at it. If you scale down vehicle dimensions, then cross sectional area is also reduced and thus aerodynamic drag is reduced. And if you are designing new vehicles, you might as well try and eliminate the sawed-off back that is common in minivans and SUVs, which should also help reduce the drag.

Not while it is true that a trivial scaling of vehicles doesn't make sense, I find it interesting that such a trivial metric actually does show continuing gains..

Isn't efficiency the amount of work extracted from a gallon of fuel. This can lead to greater power or more economy or a mixture of both. It seems car makers have given the engines greater performance while maintaining the same levels of fuel economy. They can do it the other way around.

In aircraft efficiency is a measure of specific fuel consumption that is, pounds of fuel divided by horsepower hour. The resultant figure is usually a fraction somewhere between 0.50 and 0.30. The lower the better. These calculations must be done on a dynamometer and leave out aerodynamics and rolling resistance. Ton miles per gallon can include such things as the greater distortion of tires that comes with more weight as well as the higher amount of fuel needed to reach a given speed or for hill climbing.