The problem (for me) with the "letter grade" categories is I have no idea how they translate to someone who spends most of their time riding solo-- because 20mph in a paceline and 20mph alone are two radically different things. Beyond that, I have NEVER seen a local group ride (that is, 5+ people riding the same route together) average above 18mph on Strava. I did a charity ride with about 200 people, and there were guys who ended the day with +19mph averages, despite me riding in their group for 50+ miles, and finishing 20 minutes in front of them at an 18mph average. So who knows what any of it means.