You have to restart your odometer when you get gas. Next time you get gas, record the gallons you put into and record the mileage on the odometer trip thing. Use those to figure it out. Gas mileage is affected by everything. Check tire pressure, clean or replace anything that could be dirty. Air filter, oil, oil filter, clean injectors, replace pcv valve, check for air/vacuum leaks, replace fuel filter, don't drive the car hard.
