Why does my v6 mustang Average 13.5 mpg?

there lot of answers on far. here way test fuel mileage. go gas station near long stretch of flat freeway. fill gas tank, nozzle open until pump kicks off once. wait 1 minute, swirling gas inside tank settle, begin filling again, nozzle open. when clicks off second time, stop. fuel filling complete. not top off tank again. reset mpg reading on computer, 0 , trip mileage reading zero, before starting car. must reset display, every time fill gas tank if want know best mileage. start car , on freeway, accelerating nice , easy, 60mph. hold car steady @ 60 mph can. try not drop speed or gain speed. hold right @ 60mph. not use cruise control. drive 50 miles out , 50 miles back, total of 100 miles. doesn't have 100 miles, think that's amount calculating best mileage. now, off freeway , drive nearest gas station , fill tank same way before. write down following: miles on trip meter. mpg according computer. reading on gas pump, number of gallons of gas, pumped tank. include decimals on of readings. using calculator. enter in number of miles drove, trip meter. divide number of gallons pumped. remember enter decimal point , numbers appeared on each meter. how calculated number compare car's computer reading? according fueleconomy.gov, freeway mileage should 24-25mpg. city mileage should 16-17mpg. if never reset mpg reading zero, computer readout inaccurate, week week. best way increase mpg driving @ rock steady speed in highest gear possible. in city, smoothly accelerate 35 or 40mph , hold long possible. on freeway, not drive on 65mph. , hold speed rock steady. cruise control ok, not if trying figure out if wrong mpg. repeat 1 more time, reset mpg reading zero.

i have 07 v6 mustang , ecu reading 13.5 mpg on average.
have cat exhaust system installed know should still getting better.

i'm not constant aggressive driver on throttle.

how can fix this?

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