Like Denise said - your body will let you know what speed is good for you. Some days you might want to go faster, other days you might want to take it easy. If you want to improve your overall speed, whatever that may be, then set that goal and work towards it. The more you ride, the better chance you may have to improve speed. However, you might be riding at your optimum speed now. It just depends. Riding should be fun. This discussion has quickly degraded into something not very fun. Please don't be discouraged by those who will tell you that you are slow. That is unproductive and unnecessary. Just be happy to ride and if you want to try to improve your speed for you, then I wish you luck.