A petrol gauge is a device which measures and indicates the amount of petrol in a tank. In everyday language, a petrol gauge is a measuring instrument, not a petrol instrument. 'Petrol instrument' may be used as a technical term in engineering (I've googled it and seen it on some sites), but for the average person, the phrase is meaningless. I would stick to 'gauge', if I were you - at least it is clear to everyone what this means.