Common knowledge a chip or tune will throw your overhead mpg way off. Mine was always withing .5 and now reads 3 mpg high on 80 E, and about 1 mpg off with 60 tow, I haven't checked the 120 race. 13.5 still seems low even for a EX. I get 15.5 around town/country driving and that's with DRW 4:10's, what gearing do you have? Have you checked air pressure in your tires? This can make a huge difference, try running them up near the max pressure limits. These PSD's vary a lot on MPG maybe you got a thirsty one?
NCH