My 351/C4 93 Ranger averages about 16 mpg all summer, I wouldn't accept 17-18 from a 2.9.
Tires will affect both mileage and handling but they are a compromise. Aggressive all terrain, off road, or snow tires have more rolling resistance than an all season tire- and there are some all seasons built for reduced rolling resistance. My son runs W rated, summer only tires on his Fusion sport and they handle great but they weigh a ton and are marginal in rain. Alloy wheels are generally lighter than steel wheels, less weight means less power needed to turn them. The 15x8 Weld Draglites with 235/60R15's on my Mustang weigh less than the 195/70R13's on steel wheels that it came with. I hate roof racks because they make it tough to sweep snow off (that likely isn't a problem in Texas) and because they add a lot of wind resistance and noise. My 16 Escape calculates instant mileage and I keep an eye on it and try to avoid doing anything that makes the mileage drop. A cheaper alternative for you would be installing a vacuum gauge- the higher you can keep the vacuum, the higher the gas mileage you'll get.
The guys who mentioned keeping running 100% are spot on. Old spark plugs will hurt mileage. With EEC Iv, a dirty air filter won't hurt mileage because the O2 will see it's running rich and reduce the fuel. The you'll have to apply more throttle and that will reduce mileage. A less restrictive muffler might help but avoid making sound like a fart can, please.
Lowering it might help mileage some but it will take a long time to recoup the cost with gas savings.
If you can find gasoline that hasn't been polluted with alcohol mileage will improve. My own experience showed a loss of mileage just over 5% burning E10.