What is the real deal as regards mpg on a p38 4.0?

Also any tips for a new owner about getting the best mileage from a tank.

I could never manage better than 17mpg and that was on a good day but I do have a big right foot, with boots made of lead.
That's why I converted mine to LPG.
Best advice would be to drive slowly and use the gearbox manually but then that would just spoil the pleasure of owning a V8, we will probably be the last generation of motor vehicle owners who will have that honour.