Depending on your strength and fitness level, something between 68-75 gear inches. You may have to push up a hill. I rode from Vancouver, BC to Seattle on a 72 gear inch fixed gear and had to push up one nasty hill on the last day. I did take my feet off the pedals and freewheeled down some hills though.If you are doing it singlespeed freewheel, I would recommend 68-70 inches since you can always coast down the hills.