Simple after-the-fact energy method.
A typical full-size car gets 30 mpg @ 60 mph. That's 2 gallons per hour of highway driving.
Regular gasoline has about 31.6 MJ per liter. There are 3.79 liters per gallon.
So it burns energy at a rate of 66.536 kilowatts, or 89.19 horsepower.
Piece of cake to figure it out after-the-fact. But I've read that it is almost impossible to know in advance, to design it into a vehicle. Although there are ways of approximating fuel economy.