What's your average?
This is a very common question. Someone inquiring of you about your "average" is looking to know how strong of a rider you are. They're creating some mental picture of how you compare to them.
A cyclist that averages ~17mph for any given ride is usually cruising at 18 to 18.5 mph in order to average 17. The math just works out that way given your starting, stopping, slowing, etc. There's nothing wrong with saying your average speed is 17 mph!
So, when another cyclist asks you about your average -- don't inflate the number. Just say what it is. A "real" cyclist will know that a person averaging 18mph on rides can ride 19 to 19.5 pretty consistently.
When you look to go from a "B" group to an "A" group ... do so when you know that your average is capable to increase. Your solo rides should be at the minimum for the "A" group. If so, you KNOW you can keep up with them; particularly in a pace line.
If your cycling goal is to reach the "A" group ... then work you way into the "A" group. Move up the cycling ladder while being true to yourself and your capabilities.
Cycle well my friends.