For some trips i want to do i have geared it up to a 15/42 tooth combo. My top speed is only 137KM/h. I have read that others are running 15/48 set ups and are doing 160. Can anyone tell me why my bike is so much slower even with the gearing change, or what i need to look at to get the higher top speed out of the 15/48 gearing. Cheers Chris.