Well there are a lot of conspiracy theories I’ve heard regarding the suppression of good gas mileage (some even with maybe little bit of merit), but my favorite is one I’ve heard a few times.
It usually involves a “friend” with a car or a truck that get’s unusually good gas mileage (figure 40mpg in a full size Truck or something), and when the owner tells the dealership how great it is they are asked to bring it in, and after they get it back they notice the throttle body has been changed, and now the car has dropped 10-20mpg to get the advertised amount.