The average speed of the winners of the Tour de france is about 40 kph (25 mph) in recent years
http://bikeraceinfo.com/tdf/tdfstats.html
That's avg for everything (flat, mountains)
That's achievable using a 39 T ring and 11 T cog at cadence of 90 (assuming a 700 c and 23mm tyre).
http://www.bikecalc.com/gear_speed
So for non pro cyclists, do you really need anything bigger than 39 T?
http://bikeraceinfo.com/tdf/tdfstats.html
That's avg for everything (flat, mountains)
That's achievable using a 39 T ring and 11 T cog at cadence of 90 (assuming a 700 c and 23mm tyre).
http://www.bikecalc.com/gear_speed
So for non pro cyclists, do you really need anything bigger than 39 T?