Originally Posted by
no1mad
Using Sheldon's
gear calculator, you should be able to hit a max speed of 22.3 mph @ 80 rpm.* It could be your equipment, but
I suspect if you swap out to a taller gear... you still won't be as fast as you want without working harder for it.
* I used wheel size of 700x38, crank arm length of 170mm, and your highest gear combo of 48x14.
I just double checked my own stats (26"x1.5, 170, 42x11 and 80 rpms. I come out at 22.6 mph. I'm very close to hitting that at a constant when I put out the effort, but no way can I actually hit the max speed of 33.9 mph, which would require 120 rpms.
Just for fun, I re-ran the figures, but swapped you out to a 48x11. Again all other values are the same. New potential max speed is 28.4 mph... so swapping out cassettes now when you aren't even maxing out what you already have would just be a waste of money... unless of course you want to go faster pedalling down hill.