How many miles does your tranny have on it ?
My truck had 300k+ and was getting over 17 around town.
I turned up the IP and installed a free flowing exhaust, started pushing 30psi, and a couple of trip on the 1/4 mi....with the changes ( and a heavy foot ) I was getting over 15 before the tranny started slipping..
It started slipping to the point that my mileage was down to 13 - 13 1/2..