Welcome to the club. I'm assuming you are talking about the Supercharged S (r53). From what you describe it could well be the clutch-release bearing as this is a known problem.
With regards to fuel consumption it certainly does fluctuate. The On board Computer(OBC) calculates the range based on how you drive the car. You'll probably find that on long motorway trips your range will improve considerably... Town driving will see it plummet
On my modded S I usually see on average about 300ish on the OBC when filling up.