For a while now, I've tried to come up with a common sense and simple way to explain bicycle gears to new riders (not that I'm all that experienced myself). Bicycle gears is something that I learned the hard way through trial and error long ago. It's something my leg muscles and my hands have learned to coordinate without much conscious effort in my part. For that reason when I'm asked on what the proper way to use bicycle gears, I have a tendency to utter the word "um" over and over without saying much in between. jm2, at the Bicycles Stack Exchange, put into words what I have failed to explain many many times.
Don't overthink it. Since you've got a triple, you're probably right to be in your middle ring most of the time That's normal.I know it looks a little out of context this way, to get the full conversation please go the original post. And again, I couldn't have put it better into words myself.
Regarding your hunch that you've got a lot of overlap between gears, you're absolutely right. Think of each ring as a gear range (climbing for small, normal use for middle, ass-hauling for big). If there were no overlap between the chainrings you would either A) have far fewer gears in the back or B) have a range of gears in the big and small rings which were far too small or far too big to be usable.Think about how big the jumps between gears would be if there was no overlap at all! Trust me, it's not what you want. Typically, you'll find yourself Shifting the front in one direction and immediately afterwards shifting the back in the opposite direction so that it feels like you've only skipped about a single gear in either direction. In time, you'll come to appreciate the overlap between gears in the different chainrings.
- In the middle ring you should have access to the whole cassette/freewheel in the back, though you might get a little extra noise as you approach the extreme gears in either direction.
- You'll use the big chainring when you're going downhill or your on flat ground and in a hurry. Stay out of the biggest couple of cogs in the back when you're in the big ring up front. This causes extra strain and extra wear on the drivetrain which can result in premature failure.
- Your little ring up front is your climbing ring. Use this for hills, or as your bail-out ring when you're totally out of energy. Just like big-big is a no-no, so is small-small, so stay out of the smallest cogs in the back when you're in the smallest ring in the front.
Finally, if you really want to see how much overlap you have, you can divide the front rings by each of the back rings to get a ratio for every gear combination. There are fancier ways of doing this, but for your purposes this is the easiest and most straightforward.
EDIT: Here's a pretty good article on Wikipedia about bicycle gearing if you'd like to get more in depth.
Be Safe and Be Green.