That was exactly my reasoning. I picked up an aristo TT VVTi for about a grand cheaper than market value with some nice mods on it so I bit his hand off and factored the gas conversion into the price. The more expensive petrol gets the quicker you'll get the money back. I'm now saving 65 pence a litre over SUL. Just over 10k miles to cover the cost. I'm 3/4s of the way there already......... I don't justify it that way though. I justify it with I couldn't run the TT daily at the cost of fuel now so I'd have a sh*tter to run instead. The annual saving on tax, MOT, insurance and other running costs of older unreliable motors is saving enough, and I get to sit in my nice car daily