Here is the
link to my car on Fuelly. It basically just gives you a way to track your mileage. Each time you fill up, you enter your odometer reading (total, not trip) and the volume of fuel you put in, as well as price. It then looks at the distance since your last fuel up, and does the division for you.
So it's nothing revolutionary in the calculation, but the cool thing is that it allows you to view other people's information. So, I can check out everyone else with a '15 Golf TDI and compare my mileage, or any other cars for that matter.
Of course they have an app, and forums and other content, but the basic principle is crowdsourcing MPG information.
It allows you to indicate your city/highway percentages as well, which I try to keep accurate. I also include a note with each of my fill-ups, which says what the car's computer calculated MPG was for that tank.