There's also a lot more to group riding them simply power and weight. Are you drafting effectively (i.e., between 6 and 12 inches off the next rider in front of you) and are you allowing gaps when accelerating, decelerating or doing corners? If you allow gaps, you're going to end up with all the wind in your face and you'll have to work twice as hard. 17-20 MPH in a group isn't much at all, really, especially if you aren't pulling.