They should start reporting.....
.....Miles per BTU burned per pound of mass driven.....
as in including all electric power plant BTU input before conversion and and transmission losses and a test to show performance during acceleration and deceleration. This would be a true measure of the power plant to axle effectiveness from fuel to driving a car regardless of weight.
Hybrid does gain efficiency due to regenerative braking (if it is there), diesel gains efficiency over gasoline I think, but "plug in" gives the advantage of using grid power unless the measure accounts for it. The mass factor would add the ability to directly measure the power train without giving credit for a ridiculously light car driven by a 90 pound weakling.
I don’t believe diesels are more efficient. Diesel fuel has more BTUs per gallon than gasole and therefore more miles per gallon. It makes use of a more powerful fuel.
That’s a good idea. You’d still need city/highway because the mass of the car has little to do with highway mileage.
OTOH, most people wouldn’t be able to understand the measurement.