I just started using Fuelly.com I've only filled up once so far so I don't have any long term experience with it. You can look to at vehicle type to compare your usage to similar vehicles. Just cruising around the site it seems to have promise.
It has a mobile site that renders on any phone, my experience with that that for a the first try was easy and it worked well.
This is also the one I've been using, it's great! The webpage is optimized for mobile devices, it's super easy to enter the info in quickly, there's room to put notes if there were special circumstances surrounding that tank (towing or whatever), and it graphs your results so you can easily spot trends, etc. Also very easy to convert from MPG to L/100kms, and even into UK MPG. Worth checking out.
