What was the formula for determining cars fuel efficiency based on?

Don S asked:

its weight, speed and coefficient of drag? For example, adding a hundred pound to a cars weight will reduce gas mileage by a mile per gallon. Reducing speed from 65 to 60 MPH increases miles per gallon by fifteen percent, and an SUV of the same weight and running at the same speed as a sedan has greater coefficient of resistance (Cd) thus lower gas mileage. Therefore since vehicles cannot be too aerodynamic due to effects on visibility and lift, it has only the weight and the speed that can be adjusted to increase fuel efficiency. Speed and acceleration are based on the Horsepower to Weight Ratio. So for car manufacturers to achieve the government mandated fuel efficiency they have to either reduce HP and/or reduce weight. Most customers are not going to buy slow cars. To get weight down car chassis will have to be made of (used to be) steel, down to plastic, down to aluminum, maybe wood or cardboard, and if you wish to spend the money carbon composites much like what they use on the new Boeing 787 Dreamliner.
I’m referring to the various normal day to day driving conditions which affect gas mileage. Because the vehicle is static during the test, I don’t believe that the dynamometer accounts for the wind resistance as the result of speed.