I lived in Australia for a few years and I knew that they sold gas (LPG), diesel and petrol in stations.
I am not a car expert, but it seems that when you buy your car in Australia, you need to ask if the car runs on gas, diesel and petrol.
When it comes to English, in many countries, people say "fill up with petrol" but American people might say "fill up with gas (petrol)".
In the USA, do you say "my car runs on LPG gas" or just "my car runs on gas" if your car runs on liquefied petroleum gas (LPG) or autogas that is liquid compressed from gas air used for cooking?
Some people in the USA say "my car runs on propane" but "propane" doesn't sound like an easy word.
Best Answer
As the 'G' in 'LPG' stands for 'gas', to say "LPG gas" would be a tautology in British or Australian English. In American English, it would just be confusing.
The American use of 'gas' for what British and Australian English speakers call 'petrol' is an abbreviation of gasoline. Other territories call LPG 'gas' because it actually is a mixture of hydrocarbon gases such as propane and butane, in liquid form. So to call 'LPG' a type of gasoline would be incorrect.
You should just refer to it as 'LPG' (or whatever other term seems to be used in the area you are living in/visiting - comments have suggested that some Americans refer to it as propane or autogas... it seems like nobody can agree on what it's definitely called) so as not to confuse it with gasoline. Similarly, you wouldn't say "diesel gas" - you'd just say "diesel".
I have found that some people use "petrol" or "gas" as an umbrella term for all kinds of fuel - in the UK and the USA places that vend fuel are called petrol stations and gas stations respectively. But most people will use the specific term for the kind of fuel their vehicle uses, to avoid confusion and for correctness. If you want an umbrella term for all these things then use 'fuel'.