Eternal Dilemma

I am hopeing that someone can help with a problem i have , I am looking at buying a 2004(54) Range Rover .. either diesel or petrol but i have been told he petrol is heavy on the MPG and keeps changeing up and down the auto box all the time and the diesel is slugish and unresponsive , which is why i have the dilemma , if i was to get the Diesel would chipping it give it more poke , or would it then give it the same MPG as the petrol ....... as you can see i could do with some help

Depends on how you drive: if you're getting close to the average combined fuel consumption in your current car, then you could expect to get the same from an L322, which would be 25mpg for the diesel and 20(?)mpg for the V8.

So for a 25% fuel saving of the diesel will cost you 13% more in fuel costs, plus you'll be spending £600-1500 on chipping a diesel, so that means payback in 18,000 to 33,000 miles.

Makes me wonder why i bought a diesel...