Topgun, what is your fuel consumption when using petrol?

I know that it is normal to lose around 20% of your milage when using LPG. This is due to the fact that LPG has a lower energy content compared to petrol. http://www.boostlpg.co.uk/what_is/faq.htm#3 However, this is more than made up by the price difference between petrol and lpg.