'Gear Inches' is the equivalent diameter of the drive wheel on a high-wheel bicycle. When chain-drive 'safety' bikes came in being, the same system was used, multiplying the drive wheel diameter by the sprocket ratio. The formula is:

For the sake or simplicity, we'll call the diameter of most 700c & 27" wheels '27"'. So, if you had a 42-tooth chainring and an 18-tooth cog, your gear would be:

OK, so now you know how to figure out how big your gear is, what combo do you use...? Good question. Keep in mind that since you can't coast on a fixed gear bicycle, you'll need a gear that will allow you to not only climb most hills but also to DESCEND most hills. If you have never ridden a fixed gear bicycle, this latter issue will be a bit hard to understand until you actually get out there and have to go down something steep. All of a sudden it's a whole new ball game!

Experience has taught me that the above combination of chainring and rear cog is pretty darn versatile for riding around town. Remember, NO gear will be 'perfect' for the flats AND the climbs AND the descents. That's part of the fun of riding a fixed gear bicycle. A 42x18 will allow you to ride comfortably at 18-21 mph on the flats, climb most hills without busting a gut and once you get a few skills, descend most hills without grabbing a handful of brakes.

One other issue to consider is the gear that everyone else will be using. This gear (42x18) just happens to be the most popular combo where I live. Most other people on a fixed gear will be riding the same thing. This is invaluable when you go out for a group ride on fixed gear bikes. And, tons more fun! Which, after all, is what this is supposed to be about...

ASIDE - this gear is so popular in my circle of friends because it's not too big. The people I hang out with are mostly racers that want a change of pace in the off-season. This size gear is like a governor of sorts in that it prevents you from going too fast. No mashing/hammering allowed; it just isn't possible. I know of some Randonneurs that use larger gears for long rides but they don't race. And, it's pretty hilly where I love so if I were to go much bigger, I'd grind to a halt on the climbs and tip over. No fun that.

So, take my advice and get a 42-tooth chainring and an 18-tooth rear cog. If, after riding this for a while you decide to switch, go for it. But, part of the benefit of riding a fixed gear bicycle will only be realized with a 'medium' sized gear like this. See the training page for more on this topic.