My bikes have "bailout" gears as low as 45", but I rarely need them, typically I only need to go down to 55-60" or so for hills. My old school hardware of 52t large chainring and 13t small sprocket limits me to a 108" top gear, that is usually sufficient for applying power downhill, except with a tailwind, then again, 30mph is probably fast enough (with 25lb+ commuting payload), I'm content to coast at that point.