I get 24mpg on the highway, and about the same in the city.
Whenever I see or hear folks making statements like this I always think they don't know how to check their fuel economy accurately.
I have owned over 100 cars. I always check the fuel economy using the proven methods of getting accurate calculations. I have never had a vehicle which got the same fuel economy around town as on the highway, nor have I ever had a tow vehicle which got the same fuel economy towing as driving without the trailer.
I have changed differentials and gear ratios on at least a half dozen vehicles and through the years. Putting in a taller differential has never failed to give better fuel economy. On my diesels it is pretty much equal to the difference in gear ratios, on gas engines it is not as strong a relationship.
People say that it takes the same energy to run the car through the air and down the highway no matter what rpm the engine is turning. That is true but no engine is equally effecient under all rpm and load conditions. One day with our computers and so forth this may become true, but it is not true yet.
In recent years multiple gear ratios in cars have gone through the roof. the newer benzes are running (what is it?) Seven speed automatics now? The additional ratio choices makes it much easier to achieve both good acceleration and good fuel economy.
If it were true that a car uses the same amount of fuel no matter what rpm the engine turns simply because it only takes X amount of energy to go X miles per hour all cars could simply have one gear and the ratio would not matter much.
