Sometime, when you get a chance to watch the Tour de France, first, watch the course explanation for the day at the beginning of the stage - it will show a course elevation map showing the hills and their relative elevation over that race stage.
Then, scope out the rear wheel gear cluster you'll see on the rider's bikes (they always show them if you look closely). For those days when it's relatively flat, the rear gear clusters are tiny, meaning that they are geared relatively higher (smaller numerically) in overall range so they can go faster with the less effort needed for flat running AND don't bother with gears for the hills they won't see. It's not unusual for 45mph sustained for the peloton on the tidal flats of Brittany (with no tail wind). Any crashes at that speed are spectacular, to say the least.
On days when there are a lot of hills (and particularly when it's very hilly), that rear wheel cluster will be a whole lot bigger, giving the riders the power needed for the hills, but the overall speed might average 10mph or less.
Recreational riders typically have a pedal cadence in the mid 70's. Very fit riders and casual racers tend towards the mid-upper 80's. Lance Armstrong and the US Postal Team developed the 90 rpm cadence as an art form - they would hold that pedal speed throughout the entire race stage and, as a team, were like a freight train (of course, drugs might have helped, but you had to admire their power at the time).
I've got a couple of rear wheels and can swap a cluster (or individual gears) in a few minutes and everything Stan mentioned is right on - Getting the gear splits right, especially on a bike with 22+ gears to choose from - so there's no overlap or big gaps between gears - is crucial to effort-less riding. Most of the time, I ride the same split set as I'm used to it, but if I know I'm doing a day of hills I might pop in a set for an overall lower range. It makes the day a lot easier.
I wish it was that easy to change the Speedster gearing, but you've got a bit more power to deal with to cover weird gear splits - and a MUCH wider power band!