Pros and cons to this. You have to keep the car clean before you put it on. In wind over about 20 mph I'd seriously consider taking it off. Make sure it's well strapped down with bunges. And make sure you get one with a Tyvek coating on the inside. I believe its a PTFE coating with does scratch but you will be talking certainly over £50.
http://www2.dupont.com/Tyvek/en_US/uses_apps/covers/car_covers.htmlHe is mine which I only use in the middle of summer if the sun strength and temperture is getting very high.
