Well if you want to trade fuel economy figures, I did a recent trip to France, where I ran for approx 90 miles along a dual carriageway at a steady 60mph. The fuel computer turned in 22.1mpg.
Now here's the good bit - that's running a 4.6 on LPG (multi point, fully sequential system) at 45.8 pence per litre. Based on the typical 104p for diesel, this means financially I was running at about 50.2mpg.
Now this is a bit extreme as no one drives everywhere at a constant 56, but I averaged 16mpg over nearly 5K miles according to the pooter, so on LPG at approx 50% cost of regular fuel, that's averaging 32 to the gallon. Makes the diesel look positively BAD.
Poor fuel economy AND performance, why even consider it.