Not sure what is in the tutorial, but here is something to help.
For gears practically any ratio of values you choose will be equal the the ration of another value.So to determine the no. of teeth on one gear you could make the relationship. T=No. Teeth and D=Diameter.
T1/T2=D1/D2
Solve for T1 you get:
T1=T2*(D1/D2)
Let's test this:
If you had a 2 inch gear and a 4 inch gear, then you should have half as many teeth on the smaller. Let's say you have 40 on the larger then we should expect 20 on the smaller.
So let's plug these numbers in:
T1=40*(2/4)=20
I assume you will also need to know the rotational relationship of the gears to make an expression to drive one of the gears. So lets make a relationship with the rotational value. For this we will assume that a smaller gear half as small will rotate twice as fast.
So Rot1/Rot2=D2/D1
Solve for Rot1
Rot1=Rot2*(D2/D1)
Test: Assume Rot2=360 degrees (1 turn) the smaller gear should turn twice or 720 degrees.
Rot1=360*(4/2) =720
Note: You might have chosen a relatoinship of Rot1/Rot2=D1/D2 but these could not be equal, since by common since we know that Rot1 must be bigger than Rot2 so you have a value of something bigger over smaller = smaller over bigger. Not sure if that makes since. If it doesn't just right down the two equations. Or do what I do, pick a ration and test it. If it is wrong invert one side of the relationship.
Hope this helps,
Urp
