Just fill the car's tank up and run it for 100 miles and then refill the tank to full again. Check the miles run and fuel amount added and then check the gauge position before and after the fill and calculate the Miles per gallon. This manual check will show if the fuel gauge is not reading accurately and if you are burning excessive fuel. If you are burning excessive fuel have the engine scanned as the computer may be generating some fault codes which will help point to the possible causes of the excessive fuel usage.