Join Date: Aug 2007
Location: Northampton, MA
Bikes: Iron Monkey: a junkyard steel 26" slick-tired city bike. Grey Fox: A Trek 7x00 frame, painted, with everything built, from spokes up. Jet Jaguar: A 92 Cannondale R900 frame, powder coated matte black with red and aluminum highlights.
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Some quick terminology:
Chain ring: gears in the front, attached to the crank.
Sprockets: gears in the back, attached to the rear wheel.
Crank: the arm that connects the pedal to the chain ring.
Kiester: Yer butt.
OK, so the deal is, the bigger the difference between the circumference (measured in teeth, since teeth are all the same size) of the chain ring and the sprockets is what makes the acceleration higher or lower and the top speed lower or higher, respectively.
If you're using the top four gears, therefore, there are two ways to change that: put on a larger chainring or a smaller set of sprockets. I'm on a super-crappy internet connection right now so I can't check to see what your gear ratio is, but my guess is that you've got something like a 48 tooth big chain ring and a 12 tooth small sprocket. That's a gear ratio of 4:1. If you're riding on flat roads all the time, you probably spin out pretty easily, so you'd want to put a big road chain ring on there — something like a 52 tooth ring.
Since I don't know much about your bike, I can't offer any really specific advice, like what chain rings would fit, or even if they're removable (because if they're not, you'd have to change the cranks, too). But that's pretty much what you need to do.