Re: 15 MPG eeesh...
Other factors to consider are replacing some emissions components, such as O2 sensors, and even the cat converter, especially if there is 60+K on the vehicle, I'd point more toward bad O2 sensors. Normally, a check engine light will come on to notify you, but not always. Are you taking this 16mpg off the computer, or hand calculating? Computers can be off slighty. I found all of mine to be off by up to 10% in some cases. If you idle the vehicle for prolonged times (more than five minutes), it'll surprise you how fast you'll eat up fuel. Especially in drive-thru lines. If you have 50K or more, I'd also suggest getting a tune up done, more so new plugs and to make sure they're gaped properly. Remember, EPA tests vehicles on the highway cycle at 55mph. SO driving perpetually at 70mph or greater will result in worse mileage by up to 25% and closing in at 35% at 80mph. If possible, don't drive above 62mph and see how you do for mileage. If you have an AWD, it might also benefit to change the axles, transfer case and engine oil to full synthetic fluids as it'll free up some HP by lessening friction on moving parts.
'11 Ram Quad Cab 4x4 SXT 5.7L HEMI (stock); 48,500m
'06 Pontiac G6 GT 3.5L 220hp; 64,000m
'12 Chrysler 200 Limited 3.6L 283hp; 28,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!