assuming a 700x23c tire:
x=chainring teeth count
y=cassette teeh count
z=cadence

x*z*60*2.096/(y*1609)=speed in mph

to see the difference i would plug in your typical cadence and see what speed you would go in the upper and lower limits of each. really though, a majority of your time will be in the middle gears. i wouldn't go for an 11-26 unless i was running 10 gears.