Google Maps has added a fuel calculator widget to the bottom of your driving directions, at least in the UK.
For example, if you are driving from Manchester to Edinburgh and scroll down to the bottom of the driving directions you will find a link to the fuel cost estimates. If you click on the link, it will let you adjust the price per liter.
Here is a picture:
This is currently not available in the U.S. but I suspect it might come soon. Google has not yet announced this feature, nor do I see this feature in the “labs” are of Google Maps in the US.
Hat tip to Google Maps Mania for spotting this.