We wanted to come up with a very simple way to track your mileage: each time you fill up your car you note your odometer reading. That's it. So there's no starting odometer reading, just readings at the pump.
You don't get any mileage info the first time, but after that first fuel-up you start getting info. Here's some more info from the FAQ that might help:
Why doesn't my fuel-up have a calculated MPG (L/100km)?
The exception is if you're adding partial fuel-ups. You don't get any mileage info for
partial fuel-ups, and here's some information about that:
Why do I have to fill the tank all the way up every time I buy fuel?
If you're not filling your tank you should mark it as
partial. Once you start adding full fuel-ups you'll start getting mileage info. If you usually don't fuel-up to full, this style of tracking won't work well for you.