Not quite right, 'ooman.
The calorific value (energy content) of LPG is about 5% less than petrol (it varies, some companies mix a little butane in with the propane), so you will get 5% fewer miles per gallon. The octane rating is a measure of resistance to ignition and LPG is better in that respect than petrol: you won't get 'pinking' from LPG going up a long steady incline whereas you might on petrol.