It's pretty easy actually. Figure out the size of your wheel in inches (i.e., the roll out). I know mine in millimeters since I had to measure that for my cyclocomputer. So then I just convert it. Once you have that number (for my MB it is 1988 mm, or 78.27 inches) then you just find the gear ratio.
The gear ratio is the chainring size divided by the cog size. So if you are in your largest chainring and smallest cog you'd divide 49/11 = 4.45. Multiply that number by your tire size and you get your gear inches. 4.45 x 78.27 (in my case) = 348.30 gear inches. Which means that my bicycle travels 348.3 inches for every full revolution of my pedals.
EDIT: Sorry, 348.3 inches is the distance per pedal revolution. Your gear inches is the gear ratio multiplied by the diameter of your wheel. So if you have a 26 inch wheel, and your gear ratio is 4.45, then your gear inches would be 4.45 x 26 = 115.7 gear inches. Sorry for the confusion.
From that you can expand to find your speed at a certain RPM (80 pedal turns per min, etc).
If you just want to figure out how to translate from what they say (53x11) to what you have on your bike, skip the wheel size and just compute the gear ratio. Obviously, you will not be able to match a 53x11, since your biggest gear is a 49x11. But in another example, say the book says 39x17 and you need to find a similar gear on your bike. Take 39/17 = 2.29 gear ratio.
You will need to figure out all of your gear ratios... perferably in a table format. So you'll need to know all the cog sizes in your 11-32 cogset. You'll start with your first chainring (49 tooth) and divide it by each of the cogs (11 tooth, then 12 tooth, etc., etc) and put that in one column. Then move to the next chainring and fill that column and so on and so on. Since you have a 38 tooth chainring, you'll need a cog with between 16 and 17 teeth to match that ration above (2.29) (38/16 = 2.375 gear ratio and 38/17 = 2.235 gear ratio). Or you can try your 49 tooth chainring and put it in a cog with about 21 teeth (49/21 = 2.33 gear ratio). Or you can try your 28 tooth chainring and put it into your 12 tooth cog (28/12 = 2.33 gear ratio).
As you can see, you can get really close to their gear ratio using different chainring/cogset combinations.
I don't think most people tend to work all this out before a ride. At least I don't. I just ride in the gear that lets me keep my cadence (RPM) at what I'm trying to ride. Somedays I like to do a high cadence (100 is high for me) and other days I like to just ride at my normal cadence (the cadence that I naturally do when I'm not thinking about cadence) which is 78. And sometimes, like on hills when I'm trying to work my legs more, I'll bring my cadence way down (65 to 70) and go up in my gears.
good luck,
PBW
Last edited by P. B. Walker; 11-22-02 at 12:05 PM.