As a kid the American usage 'gas' for petrol annoyed me for the literal-minded reason that petrol is a liquid not a gas. I knew that 'gas' was short for 'gasoline' (Wikipedia has some interesting little factoids on the derivation and etymology of the word: it seems it caught on in American because, in the late C19th, 'petrol' was a trade name) but that didn't stop my first reaction being annoyance upon hearing -- on film, for instance -- petrol being called gas, on the grounds that petrol is a liquid not a gas.
Now I've changed my mind. The issue isn't one of absolute definition, after all, but simply timing. Petrol is a liquid when you pump it out at the petrol station; but it's a gas in the engine as it makes the car go, and that's when it's most itself. So maybe 'gas' is better.