Everyone here will probably say to get gauges first, and it is a good idea. However, if $400 is all your are going to put into it for a long time, then I would get a tuner or chip from a reputable source, and keep it set at around a +60HP setting. When your towing heavy, you probably need to back it down a little, or at least exercise a bit of caution in order to avoid high egt's. As for the mileage increase, I wouldn't expect much. Maybe a 1 mpg increase if your lucky.