I think you'll find most folks with that 3L engine coupled to an automatic trans get 12-14 --- 16 MPG at best. Add 200k (worn rings, valves, injectors) plus pehaps oversize agressive tread tires and you're looking at 10-12 MPG. If larger than stock (225/75x15 or the 31") are on it then the speedo/odo aren't even reading accurately - you could check with a GPS unit. The later Yota engines - 3.4 or 4L aren't an easy swap. Some folks have done a GM3.8 or 4.3L swap for about the cost of a rebuilt 3L -- if you can do the majority of the work yourself. A lot of the inefficiency is in the AT used up til 96 or so too (not just the engine).
A rebuilt engine, new headers w/slightly larger diameter exhaust, calibrated fuel injectors, stock road tires, and better air intake could help and not break the bank. Oh, and dump all the extra accumulated weight in the back and press easy on the gas pedal!