Excuse my maths on this one, how is the calculation worked out?
1) Assume 20mpg (I think lpg returns slightly less mpg?)
2) 20mpg = 20/4.54609 = 4.4 miles per liter (in English, 'I do about four and a half miles for every litre of fuel I use')
3) So 40,000 miles = 40,000/4.4 = 9,090 (In English: 'over 40,000 miles I used a bit over 9,000 litres)
4) Say petrol has 'averaged' £1.60 a litre at the pump over the 40K miles, then £1.60 x 9,090 litres = £15,000
5) Say LPG averaged 80p over the 40K miles, then £0.8 x 9,090 litres = £7,280
6) Which is a difference of £15,000 - £7,280 = £7,700 saving.....but that assumes super unleaded 'normal' petrol gives the same mpg as LPG, but I think LPG don't give as 'good' mpg as super unleaded petrol? If so, and say LPG gave 16mpg, the savings would be 'only' around £6,000 or so?
Good savings anyway, given an LPG conversion costs, what about 1.5 grand?
(the 4.54609 converter used is the conversion factor for (UK) mpg to litres)