A really stupid thread, it would seem.
 

I'm the last man out of the cave.

Because tires and wheels are different sizes so simple ratio doesn't provide as much information. Some would argue that gear inches don't provide enough information either.

makes it easier to compare bikes with different sized wheels without having to do very much math

it's another way for people to be vague.

right, I think you are.

why do thermometers have numbers on them?

How else to make a schematic comparison of ,say 44x17 and 50x19. Which is bigger? How would you measure if not in distance travelled?

well if the wheel size is the same it's easy
2.58 vs 2.63

Other people obsess over it? Seems you're the one changing your gearing all the time.

Its a way of expressing how easy or hard a gear is. Since there are many equivalent gears, for example a 51/17 is exactly the same as a 48/16, its easier to talk about gear inches, which is the number of inches the bike travels with one revolution of the pedals, than it is to talk about specific CR/cog combinations.

In general, a 70 inch gear is a pretty easy around town gear for the flats. The 42/16 gear you started with is about 70 inches. Track racers usually ride something in the 88-92 inch range. Your 48/15 is about 86 inches, so you are getting up there with the track racers.

Easy to calculate, Cr teeth divided by cog teeth times 27 gives you gear inches.

There's nothing vague about a system that measures exactly how far down the road your particular set-up will carry your butt for each turn of the crank.

All this means nothing without tyre/wheel size factored in. hence gear inches.

and some will say that gear inches is useless without taking crank length into account. hence sheldon's gain ratios.

you=wrong.

That's not exactly how gear inches work and it is still vague compared to gain.

except gear inches doesnt equal distance traveled per crank revolution

circumference= Pi * diameter

gear inches = gear ratio * wheel diameter

actual distance traveled per crank revolution is gear ratio times circumference

a 72 inche gear is equivalent to riding a highweel bike with a 72" front wheel.

read this: http://www.sheldonbrown.com/gain.html

oh sorry, was i being vague?

One valid reason I can think of is going between offroad 26" and 29" wheeled bikes. In order to keep your gearing consistent, you figure out the gear inches and keep that the same.

I agree with you for on-road stuff though, not many folks go between 650c, 650B, and 700c at all.

Totally OT, but you know that you spelled "Szyslak" wrong,
right?

You're both wrong

http://www.landofthelost.com/images/lilenik.jpg

Sleestaks

But people do go between different tires often enough. When it snows I put a 32C xcross tire on my rear wheel, which increases the gear inches of my bike.

I need at least 80 inches to stay with my Sat morning geared group ride.

while we're on the topic:

can someone sum up the advantage/disadvantage (if any) of 2 different combinations that have the same gear inches?

Ahem.