you should really clarify that, because gearing has to do with your wheel size too. if ive tried 48/12 on a 12 inch kids bike my experience wont matter (not like it does anyway)

are you trying to go as slow as physically possible on a bicycle? youre certainly not averaging 40 mph in nyc. only if the entire city was a ghost town would you even reach > track speeds in the streets

why do you want to use such a high gearing in the first place?