18-20 MPG seems very low. I have an 1984 Talbot express camper conversion with the 1971cc petrol engine. It has a longer wheel base than the standard Talbot Express camper of the same age, and has a high roof. I average 27 to 28 MPG, which is is not too bad for a 22 year old petrol engine. It's not great on very steep hills, but will sit at 70 all day on the motorway, and is quite able to do a good bit more on the flat. Could the engine not be tuned up to improve on the MPG, rather than going to the expense and work of converting to diesel?
John, September 2006