Mark put it perfectly. I would recommend all the same things, in that order. Do one, then wait, then do another.
Air pressure is a mileage killer. I usually keep anywhere between 28-32 psi in the rear (without heavy loads) and 36 psi up front.
I'm supercharged and getting 18.6 on highway and about 16.7 - 17 in the city, so there's no way you should be getting 16 highway unless you drive like a maniac! <img src="/forums/images/graemlins/drunk.gif" alt="" />

'99 Tacoma Xtra cab V6 TRD
TRD Supercharged, URD 7th injector and 2.2" pulley, 32X11.5 BFG All-Terrains, OME 881 3-inch lift/Toytec AAL, 5100 Series Bilsteins all around, and ProComp 130 lights.
No I'm not some creepy naked guy...