This site is wonderful and will take the place of an Excel Spreadsheet I've been keeping since I got my new car.
I do have one functionality suggestion that is outside the vein of fuel economy monitoring but would be nonetheless helpful.
For those of us who lease vehicles, it would be great to have the ability to track your mileage and compare it to the terms of your lease. I'd like to know whether or not I am driving at a rate that's over the 15,000 miles a year my lease allows, to avoid having to pay overages when I turn in my car.
Just a suggestion, thanks again for this excellent tool.
I wonder if this could be implemented with tagging. For example, you tag all business trips with "business", and then when Fuelly displays that tag page, it shows some aggregate statistics about all fuel-ups tagged "business" including total miles, avg mpg, etc.