If you are going by your overhead, you do know you can't trust it anymore, right? Mine was always within 1/10th of hand calc. then I added the tuner and there it went. That is the only negative to a chip or tune. I have noticed that my 60 tow is almost right on, my 80E is right at 3mpg high and the 120 race is dreamland!!!
A 5 mpg jump with just exhaust is not going to happen, there has to be other variables at work here. Fill all the way to the top of the fill neck, this takes patience, then do a hand calc the next fill up, I think you will find you will be 1-2 mpg ahead if you drive like a old man, if you like the extra power Jody gave you , you might even go down!!