Phil,
Remember that these figures are for US gallons 3.78 Litres -whoops I mean liters- rather than the UK 4.54 liter gallon. I was getting 18-22 MPG with a 350 and automatic trans. have just fitted a 5 speed manual which should make difference when i get the truck on the road in a month or so.
If you are going to do a lot of mileage in a stovebolt in the uk it might be worth thinking about an LPG conversion for the truck, there's plenty of room underneath for the tank(s).
Stuart.
