What is your actual cadence? I had no problem hitting close to 30 mph on a 42x18.
My Mirage runs a 52x16 and I can do flat sprints at nearly 35 mph and will top out
around 40 mph on a good down hill. That's with a controlled spin of about 155 rpms.
On a really good day I can manage 180 rpms and could go even faster.