I went from a '10 RT to a '13RTS a few months ago. the break in mileage on the 13 mirrored that of the 10 at about 25-28 mpg. I am now wintering in AZ and most of my driving is around the resort with a 15mph limit and trips to town at 35 mph. The first and second fillups showed an amazing 16 mpg. I filled it to the brim as we were leaving for an 80 mile freeway trip at 75 mph and the tank full mpg jumped all the way up to 20 mpg. No visible gas leak. engine running smoothly and idle at normal 1350-1400. Any thoughts?