help, mileage problem

my 1999 f150 xlt with 4.6 auto got an average of 20 mpg (imperial gallons) hwy. till about 18 months ago. power dropped a bit and mileage now hwy mileage averages between 15 and 16. the truck has regular maintenance, and has recently had new plugs and high perf. plug wires. I've had it to my regular garage and the local ford dealer, everyone says nothing is wrong, nothing shows up on the computer etc. my regular mechanic suggests electonic parts that are functioning well enough not to trip any error codes but are not functioning as they should?? this is an expensive problem with the price of gas. any suggestions???

Make sure they put properly gapped , oem spark plugs in it. Most people consider them to be pre-gapped. Ive found them too close, too wide, and some just right, always check em. Have them check the fuel pressure, a malfunctioning regulator can cause problems. A little too much pressure can cause excessive gas consumption, but no real driveability problems. And on most vehicles, it wont set a trouble code, until it carbons up the oxygen sensor, or the converter. Have them do a vacum check, if this doesnt look normal, it could indicate a partially clogged convertor/exhaust. Just a few things to check.

thanks unclebubbles. I passed all this on to my mechanic, they checked a few things, finally they cleaned the mass air flow sensor and all is back to normal. (I would have thought that might be part of regular maintenance, but it had never been cleaned)