If you look at the driving profiles that the cars are put through for city and “highway” driving, you find that the city might be reasonable, but the “highway” is run partly at 40 mph and partly at 50 mph. There is a separate but apparently unused rating called “high speed” that averages about 65 mph and is more what people think of when they think of highway. As far as mpg goes, at high speeds, resistance is mainly from air drag, and is proportional to the car’s cross-section and sleekness. SUVs lose out bigtime on both counts. The rest is from road friction and is proportional to weight, again a loss for SUVs. But the important point is that air drag is proportional to the square of the velocity at high speeds. Hence the ratio between an actual speed of say 70 mph and the “highway” rating of 50 mph is a ratio of 4900 to 2500, or a factor of two. Still, road friction comes in to lessen the ratio, but the “highway” rating should be taken in such a light, and probably replaced with the high speed rating so as not to confuse the public.
In practice, there are only two safe speeds allowed on Orange County freeways: 70 mph plus, or a slow stop and go crawl for miles at rush hour. You won’t last five minutes at 40 to 50 mph when the freeways are open. So for rush hour commuting, use the “city” rating.
