Petrol price hike

Yea, I know. Just by reading the title of this entry, u know this is old news. Anyway, I have come up with my own way to save money on petrol. I am not gonna fill up the tank to full from now on when the tank is almost empty. I'll only fill up to half tank max. This in turn will lessen the overall weight of the car and I will ultimately get more mileage per litre... I hope. Can someone do the math and tell me if this is right?