Wrong!
Most stock mustangs have 3.27 gears from the factory. I have no idea why Ford put these gears in our cars because they’re awful. Despite what some “experts” might tell you, gears will not decrease your top speed in a mustang; in fact, they’ll probably increase it. If you were to top out your car with 3.27 gears, you’ll notice that the power just stops when you shift to 5th gear. This is because you are not in the car’s power band to pull the car up to higher speeds. Not only will steeper gears allow you to pull quicker, but they’ll also keep you in your power band which will, in turn, enable you to pull stronger in 5th gear; something you could NOT do before. Other experts claim that you will lose gas mileage due to the higher RPMs that your car will now be traveling at on the highway. In reality, you’ll lose maybe 1 MPG at best.
Thats from stangnet.com if you havent already read the article you should go read it here is the link
http://www.stangnet.com/content/view/23//
Anybody debating 3.73's or 4.10's go read that article that is the real truth about 4.10's.