The "ECO DRIVE" indicator shows you trip mileage since the car was started. It's displayed somewhat annoyingly as a bar that fills up. If you get 26 mpg, one bar on the left will be filled. Getting 41+ mpg, all will be filled. It can really only show trip mileage between 26 and 41 mpg.

When you start driving, your mileage will be less than 25 mpg for a little bit, but eventually it'll increase and you'll start seeing the bars fill on the gauge.