I've been gettin consistant 13.5's stop and go...I've got a pretty big lead foot, so thats part of my problem, I'm sittin close to 2000 miles and they say "fuel mileage will improve the first during the 4000 miles". I have yet to fully test the MDS system on a long highway trip. I'm going to be going with a true dual exhaust system in the near future, and, not really expecting a huge increase, but a small increase of mileage is possible...