You say your monthly fuel expense is £120 so if you would stay on petrol for the next 3 years it would cost you £4320 (£120 * 36 months) in total
...but if you would go for the LPG conversion it would cost you £2140 for the same period, but lets say £2500 in total, so there would be over £1800 saving on fuel over 3 years, am I right???