Re: 2009 f-150 5.4 fuel milage
Depending on where you live, winter might have some effect on this. Colder start up temps, winter fuel (additives to help start the engine in colder weather aren't good for fuel economy). Has your driving style changed? With it being an '09 did you baby it for the first while and are now, even slightly, more aggressive with the throttle? Are your tire pressures good? have you carried any passengers around town lately, or something else that will add weight to the vehicle? Do you idle the vehicle to warm it up? Or do you just get in, start it, and drive away? Even short idling spurts can drown mileage numbers.
On a side note, my 4900lb Durango R/T AWD with the 5.9L engine gets 13.5mpg in mostly city driving. Pure city driving (absolutely no highway) in winter I'm lucky to see 12mpg. I didn't buy it for the fuel economy.
Longer oil change intervals mean less maintenance over time, but in my area, heavily salted roads = lots'o dust, and it can clog an air filter in less than couple weeks of daily driving.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!