that's because it affects your speedometer reading and top speed/ ride height, although I think the agility and grip of the tyres is worth the downside. It really depends what you are doing with the car. If you are just using it on the road stick with tall tyres, if you are doing track days or speed events go for low profile.
If you are doing trials you need tall grippy tires...........
Black circles.com do a good tyre chooser.....
John
