Also to add to this - gas wouldnt be usable after the first year from the apocalypse due to chemical changes in it (unless it had been sealed in a completely airproof container, cars and petrol stations dont count). Diesel is better in this matter as it can last even more than 10 years, but it has a problem with microbes growing inside it, which produce substances, that halt and damage engines the fuels is used in.

There is also an option of producing bio-diesel from vegetable oils or animal fats - it isnt a simple matter, but it is much easier than refining oil into gasoline.