Bad Gas Mileage on '89 Camry?

Toyota's of those years had crappy V6's. I realize you have put some money in the car, but 4 cylinders of those years were all around much better. Also just because the check engine light isnt on it can still have a "slow" O2 sensor. If it has more than 65,000 on the sensor or if the engine it was in ever ran too rich or burned oil it may need changing. Hope that helps and good luck.