It's free so it's worth the money
Garmin have had this in their GPS systems for years. It only knows your speed. We all know you can't base economy on speed (and calculated rate of accel/decel). If you use BSFC it will kill your score, and it can't account for EOC, DWL or P&G. My GPS app always hovers around 12.2l/100km, while actual tank to tank varies from 10-13.
Even OBD scanners aren't accurate compared to actual pulses from the injectors.
If you're on this forum you probably already know more than a simple app can teach.