I would really like to see this formula used to compare modern engines to those dating back 60 years or so. It seems that we're getting a lot more power out of engines (the V-6 power plants used in Ford Mustangs, Chevrolet Camaros and Dodge Challengers, for example) while using a lot less gas than ever before.
Another interesting comparison would be to see how the adoption of emissions controls impacted power and fuel consumption on engines from the 1970s. You could have a 400 cubic inch engine putting out less than 200 horsepower in the last half of the 1970s, so how did that compare against the same sized engine in muscle cars in the 1960s? My guess is the 1970s engines fared a lot worse due to reduced power and perhaps only slightly better fuel economy.