Commodore has poor fuel consumption

I would expect your car to return around 11.0 L/100 km, so yours is very poor as you say. You would have to allow for slightly higher fuel consumption due to wear on the engine, loss of compression etc., but I would still expect that you should get around 12 L/100 km. I would be checking the basics, like spark plugs, spark plug leads, air filters, fuel-injectors, and throttle body to make sure they are all in tip-top shape; then look at sensors like the oxygen sensor to make sure they are conveying the correct signal to the engine computer.