WHAT DO YOU, the average American car owner, think the average American car should be like? Should it have average economy in the 25 mpg range? Speed over the 100 mph mark? Easy driving, smooth, comfortable riding, and years of good service? These qualities add up to a marvelous piece of machinery, and while many of our cars have some of these features, unfortunately none of our cars have them all. However, before you give up your search for that average American car, do this: if you’re a Mercury owner, or someone who has driven other cars in search of a good all-around automobile, compare your notes with ours, and see if you think, as we do, that the Mercury combines performance, economy, comfort and appearance as well as any…