I own a shop, and have seen this done before. A spoiler if it is the correct one can in fact help fuel mileage. The one thing you have to watch for is; don't get one that is too long and sticks out any on either side of the vehicle, or it will drive funny and the rear end will get loose at higher speeds. If it is a spoiler made for the vehicle, then you will benefit from using it. Glad to help you, Good Luck!!!