I have read about Diesel LPG kits but the LPG will not replace diesel fuel. It is used as well as diesel to improve performance of the engine so with the extra power you dont need to floor it as much saving you on expensive diesel while using cheeper lpg as a supliment. In the real world you would prob do the same mpg and have to pay for the lpg as well because you would end up using this extra power (well I would anyway)