Although LPG has a higher octane rating (up to 110+) than petrol (90 – 100) and higher energy value (46,1 MJ/kg) than petrol (43,5 MJ/kg) it still has lower energy density because of the lower liquid density (0,5 – 0,58). Petrol has 0,71 – 0,77.
That is why the consumption (litres per 100 kilometres) is always higher with autogas compared to petrol. On average there is a 20% increase in consumption, but it depends a lot on different factors, so it can be lower or higher.