Like others in this post, have you done a tune up? Use iridium sparks, cap/rotor, synthetic oil, air filter, etc. Use BG44K to clean out the injectors, it works!
On my 93' GS300 I get about 20 MPG city/bumper-to-bumper driving.
Maybe check your O2 sensors. I've have not heard of any o2 sensor issues on these engines, but you never know.