Dan,

you'll get there bro. i am still working my way up to 15mph average and i am on a road bike with skinny tires.

change the tires if your not going off roadin. get some nice commuter tires at your LBS or some slicks. the rest is all up to the engine to get the speed faster. once we work on that me and you will be able to average 20mph on a bamboo bike