Check out the following link to a goverment web site, look at the charts to see the difference between petrol and diesel.
Note the little difference between TDV8 and 4.4 V8 .:crazy:
It will on the figures quoted, take roughly over 3 years to get LPG cost back, before you start to gain the benifit. :w00t:
http://www.vcacarfueldata.org.uk/search/runningCostSearchResults.asp