Although LPG typically costs half the price of petrol, it is less efficient (see below table), which means that your savings work out at approximately 40%.
Thinking £ per mile
Although each conversion is different, you can expect a reduction of up to 20% in fuel efficient when running on LPG.
Let's imagine that petrol was priced at £1 per litre and LPG was priced at 50p per litre.
A vehicle with an average of 30 mpg running on petrol might only achieve 24 mpg on LPG (Autogas).
However, the gallon of petrol would cost £4.55 which equates to 15.17 pence per mile, whereas the gallon of LPG would cost only £2.27 equating to 9.47 pence per mile.