ive read several articles that state the consumption on LPG is around 30% more than petrol, so if you can find LPG thats around 50% cheaper than petrol, its a saving of 20%, so in "theory", a petrol car that does 40mpg on petrol will "seem" to return 48mpg on LPG, but actually doesnt, if you see what i mean? , i need a sit down now, hahaha,