I have owned a 2001 gz250 and my wife and I both rode together on it all the time. our combined weight is about 320 pounds. it did top out about 65 mph though. I'm 5'10" and felt comfortable. I never had any major problems either. I did move up too a bigger bike (Honda magna vf750c) to get interstate speeds and passing ability. At your size I would also recommend a bigger bike. The gz250 may only give you 20 more mph than your zuma.Remember you control the throttle so just cause it is bigger doesn't mean you have to go faster. I do believe you would be happier that you invested in a bigger bike from the beginning. I would definitely see if I could transfer my deposit to a bigger bike. I hope this helps you out.
