I found this out yesterday and couldn't believe that I never knew about it before!
Do you ever drive into a petrol station and forget which side your petrol cap is on? Maybe you just bought it, or are borrowing a car for some reason and didn't think to check.
If you look at the fuel gauge, there is a picture of a petrol pump and next to it there's an arrow. The arrow shows you which side your fuel cap is on. Simples!
Am I the only person in the world that didn't know that, and it was all a conspiracy to keep it from me and laugh when I pull up on the wrong side of the pump? Or is it a little known fact?