You're not doing anything wrong, it's just the first fuel up doesn't count immediately towards an average fuel economy, we start measuring by difference on the second tank forward when you track by the odometer.
Your other option would be to use the trip mileage instead of your odometer mileage to track your fuel usage. This will also result in instant mileage info. I use the odometer to track mine, so all of my vehicles have 0 for the first tank. Feel free to look them up by clicking my name, you will see what we mean.
When I put in my second fill-up with a new car, it assumed all mileage was on the tank I was filling up, so it calculated ~55mpg rather than ~20mpg. How do I kick things back to just entering the odometer reading and fuel data, rather than figuring out how many miles I drove since the last fill-up?
walkera, just go to your car profile page and look for the "Edit This Car" link. Then change from tripometer to odometer tracking and save. Then go back and edit your existing two fuel-ups, entering the odometer reading for each fuel-up.