Here are some stats: 0.4-6.25% http://ecomodder.com/wiki/index.php/Rear_wheel_skirts

It's highly dependent on speed.

Small gains can add up. I had a 75 minute commute for a while and ran some MPG experiments on my truck. Adding a wedge shaped tonneau cover, a chin spoiler, and inflating the tires more boosted my MPG from 18-26. It was ugly so I took them off and got distracted before I built them again. It was much quieter and seemed to "roll" along at faster speeds.