how do chevy and buick and ford say there can can get like 20 to the gallon if they dnt know how i drive?
the factory mileage rating is specc'ed off of a standardized EPA highway/city driving simulation. all of the cars are taken through the same course at the same speeds with the same cargo load.
you'll notice that if you take a brand new car home that you don't normally get 'exactly' the same fuel mileage that the car dealer was displaying on the sticker. that's because you're carrying more/less weight, driving the car harder/easier, have more/less stop and go traffic, using higher/lower quality fuel or using a fuel with a higher percentage of ethanol in it ( very bad for fuel mileage ). it varies for everyone, the EPA mileage is just to give you a fairly good expectation.