Yeah there's a cadence calculator here which is fun to plug numbers into
http://www.velocide.com/cadence_calculator_new.html
a 700c x 23 wheel is about 26.3 inches. Running 48x18 I'd be spinning 144 rpms to hit 30 mph which realistically means I'm never going to be really spinning out on flats. Once you start plugging numbers in you realize that smaller gears aren't anywhere near as "slow" as some people make it out to sound.