well, if you're not planning on skidding, the you can just worry about your gear ratio/gear inch

to calculate your gear ratio, take your chainring tooth count and divide it by your cog tooth count. (what you have now = 40/16 = [2.5]2.5[/b])

to get your gear inch, multiply that number by the diameter of your tire. (27 is used as a nominal value that is close enough for most needs) (so: 2.5 x 27 = 67.5---which is good and fits in the general recommendation of mid to high 60s to low 70s)


finally, if you want to get your skid patch count, take your chainring over your cog count again and reduce that improper fraction to its lowest terms and the denominator is your patch count.

--there is also a little sidenote if you skid with your other foot forward (both feet) which could double your patch count and that is if the denominator(or numerator--I forget which) is odd; this isn't really something you need to worry about too much though.

to put that as an example, of your 40/16 = 10/4 = 5/2 and the two is your patch count.

if you really are concerned about your patch count, the general consensus as I see it is to get a prime number of teeth on your chainring and not worry about it.

Again, if you plan on using your brakes, then don't worry about all this and just go with it.

good luck