Nothing against Fuelly at all but I also use an app called Fuel Log Pro on my Android phone. I use them both together. FLP logs all costs associated with the vehicle - monthly payment, insurance, oil changes, car washes, anything you want to track as a cost then under stats it calculates all these costs into many different data sets like how much it costs to run your vehicle per day, per KM/Mile and so on and so on. To answer your question, Fuel Log Pro tells you the amount of days between fill ups as well as your current mpg, your average mpg, and many other parameters on the start up page where you add a fill up. I think you can check it out for free first but I use both apps. Entering data twice can be a pain but I like to see all costs and averages - not just fuel costs and averages. Cheers.
Thanks for the suggestion, Rubberdude88. We've had many, many conversations here about whether or not to make Fuelly a total cost of ownership application. We've always felt like Fuelly is better as a tool that's focused on fuel economy alone. Once you move to tracking all costs related to a vehicle, you're going to make very different decisions about how the application will work.
I'm glad it works well for Fuel Log Pro, sounds like they found a good balance. We don't think it would work well for Fuelly.