why are you so desperate to know it? Because it will mean that a jazz is better than a civic for commuting in? What is the actual purpose of you knowing the answer to the question what do you personally gain from it?
Do you ever read your posts back to yourself from to time and think 'huh'?
He's asking you a simple question about how many miles driven to equate to an £8 a month fuel saving. Thats it. You've spent more effort whinging and avoiding the question than it would have taken to say 'Oh, thats about 100 miles worth'.