I get 18mpg from my 4.6, just to give you an idea. It should say in the back of the owners manwell
 
Last time I worked it out in my 4.6 I got about 14mpg running on lpg doing a mix of both town and dual carriageway driving. So probably around 17 - 18 mpg if it was running on petrol (allowing for 20% loss in economy).
 
get 12 around town and about 20 on a run,just got 240 miles out of 60 quid of fuel after a run to sheffield at the weekend
 

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