Ours does 16-20 around town short trips consistently - MY08 4.0 V6

I have never seen anything that high unless I am out in the sand

If you are at half a tank I would fill up again now and see how much goes in - this will give you an indication if the fuel gauge is reading accurately.

Also remember the xx/L is based on injector duty cycle and opening times not real world calculation of this much missing from the fuel tank x kms travelled.

Airflow meter would be the 1st port of call if the tank really is half empty