Since the reinvention of the wheel --that is, since single speeding came back into vogue, after having been relegated to kids’ bike status for decades-- there has been a lot of discussion about set-up of single speeds. Yes, single speeds simplify things, in that there are fewer moving parts to get snagged or go out of adjustment, but the finer points of each type of drive train require special consideration, from chain line to chain ring bolts to the best cog or chain to use; but perhaps no question gets asked more than “What gear combo is right for me?”
Well Billy, there is no straight answer to this question, as every bike and rider is different, and terrain varies widely from area to area. Riding mostly flatland singletrack is a lot different from riding mountain trails, for example. This spew will not try to answer the title question directly, but instead give you a beginner’s lesson on figuring gear ratios and their assorted relatives. This is Single Speed Gearing 101. Some of you will find it a boring rehash; for you, go read another spew or go for a ride until you find something meaningful.

STRAIGHT RATIOS
The starting point for most folks, and possibly the most commonly thrown-about catch phrase in single speeding, is 2:1, which refers to a common gearing ratio, or in other words a gear combo wherein the chain ring (front gear) has twice as many teeth as the cog (rear gear), 32/16 being a very common variation. The basic idea here is that for each revolution of the crank on a 2:1 geared bike, the rear wheel will rotate 2 times. A 32/18 gearing yields a 1.78:1 ratio, and you guessed it: one complete revolution of the crank turns the rear wheel 1.78 times. Lower ratio equals easier pedaling (assuming you’re comparing ratios on the same size wheel each time), good for hills and slow technical sections. Higher ratios make pedaling harder going up hills, but increase your cruising speed with less spinning of the crank.
The 2:1 ratio came about in large part because it is a good middle-of-the-road (no pun intended) gearing for a 26” wheeled off-road bike, and remains a good starting point for figuring out what will work best for you. Thing is, that ratio became the norm when single speed mountain bikes all had 26” wheels, not to mention that ratios are good for comparative purposes but are not really all that comprehensive…they don’t take into account wheel size, which of course has a profound affect on how easy or hard a wheel turns. But ratio is very useful and we Surly folk use ratios all the time when talking to people about gear set-up.
When do you use straight ratios?
Most of the time, in my experience. Yes, there are more inclusive calculations which are certainly handier for figuring more accurately how your set-up will feel, but for the most part all these calculations are designed for comparative purposes, and, since the ring and cog teeth are the basis for all the other calculations, they work well for the simple purpose of comparing one combo against another. I use ratios most often when figuring my gearing and as well when talking to people about choosing their gear combos.

GEAR INCHES
More accurate is something called Gear Inch, which combines gear ratio with wheel diameter (this is overall diameter, including the inflated tire) to calculate how far your bike will travel for one revolution of the crank. Since almost all modern bicycle chains adhere to the ½" pitch standard (1/2" between pins), the calculation is easy to figure and (almost absurd for the bike industry) practically universal. The calculation looks like this:

Gear inch = (Diameter of drive wheel in inches) x (# of chain ring teeth)
                                                # of cog teeth


So say you have a 32/16 gearing, with a rear wheel with an overall diameter of 26"; the calculation would be 26 times 32, the result then divided by 16, yielding 52 gear inches. The higher the number, the farther your bike will travel on one revolution of the crank. This is not a straight distance conversion, only a relative 'feel' measurement (left over from the days of high wheel bikes). To get the distance traveled, multiply your gear inch result by pi (3.14).
When do you use gear inches?
Use GI when tire size in important to the comparison. If you're swapping the cog on your bike but all else remains the same, GI is not necessary. But say you've got a single speed bike with 26" wheels and you love the gear combo and want to duplicate that on your new 29" wheeled bike. GI is the way to go. The 32/16 combo on a 26" wheel gave a 52" result. But 32/16 on a 29" wheel gets 58". To get nearly the same feel, try an 18t on the rear of the 29er, as it will yield 51.6".
Make sense?

GAIN RATIO
Sheldon Brown, bike and math geek, crotchety old shop rat, and the webmaster of one of the most complete information archives concerning all bike tech old and crusty (www.sheldonbrown.com), suggests that gear inches is not as complete as it could be either and offers something called Gain Ratio, which is a calculation that utilizes gear inches and also figures in crank length, since crank length affects leverage. The issues surrounding crank length are many and opinions vary widely about what is right and wrong for crank length. So while it’s a useful and finite equation (i.e., crank length is easily quantifiable), I believe it’s overkill…again, these are really only comparisons. Sheldon rails on straight ratio a bit, saying “Cyclists who are only associated with one narrow ghetto of the cycling world frequently make do by just naming the chainwheel and rear sprocket they are using.” Besides being a whiny insult, I disagree with this assessment because I think he dismisses too quickly the usefulness of the simple straight ratio.
When do you use Gain Ratio?
Before you write in and tell me that you found Sheldon’s to be the most useful calculation you’ve ever run across in your entire life and I should be buried up to my neck in sand for suggesting that it doesn’t deserve a place in the canon of esoteric bike knowledge, consider a few other things that affect how a gear feels but which are not so easy to plug into a calculation:

1. Bar width. Not only are there arguments that wider bars can increase your oxygen intake, making you feel stronger longer (speculative at best, I think), a wider bar, similar to a longer crank, gives you more leverage when you’re out of the saddle cranking. This doesn’t directly affect the way a gear feels exactly, but it does have an affect on your ability to push a gear that may be a bit stiff, much like using a longer crank arm
2. Tire weight. The weight of your tires comes into play two ways: it adds to the overall weight of your bike, which is worthy of some consideration when figuring your gearing, but more to the point, heavier tires increase rotational weight. With more weight at the outer circumference of the wheel, it takes more effort to get the wheel up to speed (and the wheel will carry inertia better, making it roll well at speed, but also requiring somewhat more effort to slow). Recently I changed my tires from big, heavy knobbies to light weight, low-tread racing tires, leaving everything else the same on my bike, and the difference was very noticeable. The bike accelerated faster and climbed better (and I’m not talking about tread hook-up, strictly about the effort required to make it go uphill). In my example, some of the increase in acceleration can be attributed to how tread affects rolling resistance, but the major difference was weight. Try it using 2 tires of the same kind in different sizes, you’ll see what I mean.
3. Overall bike weight. Basically covered in the tire example, a heavy bike (converting your old Schwinn High Sierra to single speed, for instance) may require a lower gear than you would run on a lighter machine. Conversely, a lightweight racing machine probably can run a higher gear.

THE FINAL ANSWER (SORT OF)
The point is that not everything is quantifiable, and you can’t rely on a calculation to get you the perfect gear. Again: the terrain you ride, your level of fitness, your skill level, and even how your day is going all make a difference in which gear you’ll like. Also of major importance is what kind of bike you’ll be applying this to….a fixed gear bike with high pressure skinny tires will require a fairly high gear ratio, in part because it’s a lightweight road machine, and in part because fixed gears tend to be a bitch on downhills, so most riders opt for a higher gear, something that’s a bit stiff on uphills, but that doesn’t make the rider’s legs flail wildly coming down. In fact, fixed gear riders in general tend to gear pretty damn high, especially in hilly areas, like in the neighborhood of 2.75:1. As Surly pal Seattle Brad sez, you can always get up a hill; coming down is something else altogether. I would add, too, that you can get used to almost anything. Sure, you can do it completely wrong and end up with a gear that’s way to easy or hard, but if you get it close and you ride it enough it’ll feel normal before long. I live in the flatlands and my fixie is geared fairly low by fixed gear standards, at a 2.3:1 ratio, and it’s great for cruising around town. If I lived in Seattle I’d probably gear higher due to the downhills. Ask around, find out what gear combo others in your area are riding and come close to that.

26” wheel off-road: 2:1
700c off-road: 1.75:1
700c on road: 2.3:1

And one more thing…
One more consideration in all this is that different ring and cog combos can produce the same ratio… a 32/16 is 2:1, but so is 36/18, and so is 34/17. So why choose one over the other? Does it matter? Yes, a little. A smaller chain ring will get you better under-bike clearance, so if your riding consists mainly of off road jaunts with plenty of rocks and logs, going with the smaller ring is probably a good idea. On the other hand larger gears provide more chain wrap, that is, the amount of chain contacting (and therefore driving) the teeth of the gear.
Think of it this way: Say you have a cog with 16 teeth. On an ideal drivetrain, where the chain runs directly from ring to cog and back, with no interruptions from tensioners or derailleurs, roughly half the teeth on the cog are in contact with the chain at any given time. All the drive torque a rider produces is pulling the chain against the cog teeth…hard. On the ideal single speed drive train, where chain tension is maintained either by horizontal dropouts or an eccentric BB, this is probably never going to be an issue. But say you’ve converted a frame with vertical dropouts to SS… you have to take up chain slack, so you’ll likely add a tensioner like our Singleator. Most tensioners have a built-in spring, and often the tensioner is installed pushing the chain down, away from the cog, thereby decreasing chain wrap. These two things mean that once that pedal torque is applied, a smaller cog will be more likely allow the chain to slip forward over the cog’s teeth. In this case, we generally recommend going to a larger cog, like an 18t, and changing the chain ring to get you your comfy gear ratio.
Also, since the wear of the chain on your single speed’s ring and cog isn’t shared by other rings and cogs, as it would be on a multi-geared bike, going with larger rings and cogs spreads the wear over more teeth, so they last longer. Not such a big deal if you’re using our stainless rings, but with aluminum rings, the smaller sizes will need to be replaced more often than the larger ones. Just something to think about.

So that’s pretty much it, or at least as much as applies directly to the topic at hand. Now go ride.