My RR is an environmentalist! Is it a fuelling problem?

[dnb]

My 2001 RR Vogue has recently started doing much more to the gallon than it has done in the past.

Last tank (mainly B road and town driving) did 17.2mpg, where as it always used to do 15. The current tank is averaging 21mpg, although it was only filled up today.

I don't think my driving style has changed much.

What could cause the symptoms, and is it a problem? (ie running lean and risking detonation)

[900T-R]

How long have you had the Rangie? They may just be the normal seasonal differences.