Gearing and wheel size - given the same level of conditioning, matter - I had the standard 14 tooth freewheel on my Citizen Tokyo and really felt under powered in the highest gear - I swapped in an 11 tooth freewheel and increased my top speed about 10% - the biggest limiting factor remained the 16" wheels. I just upgraded to a 20" folder with a stock 9 speed, 11 tooth high gear cassette and feel a significant difference in my top speed already.