Originally Posted by Scottrik

Firstly--on rubber tire there is generally not a RANGE of pinions which, used with a given spur, creates a certain FDR...there is A pinion. Rubber tire diameter does not decrease appreciably which is why FDR becomes relevant. Foam tires, otoh, wear down and change the overall ratio (roll-out).

Secondly--if you've already found gearchart.com then the answer to your pinion is, literally, at the tips of your fingers. Open the gearchart creator, select Associated TC5 from the menu (which will auto-fill the transmission ratio field), enter your spur gear size (87), and request "middle" in the plot spur/pinion options. From there take a guess on the pinion size and click "Gear Ratio". Don't see 4.0 bracketed in the 87 column, or it is toward the top or bottom of the chart, go back to the entry page and change your pinion selection toward it. Guessing costs nothing but a couple seconds.

I could tell you the number, but this is really something you're going to have to do yourself. Teach a man to fish...

Thirdly--converting from 48p to 64p is simply a matter of applying junior high pre-algebra. What you're doing is comparing ratios. You have the 87t 48p spur, you want to find the same size 64p spur. Your ratios look like this:

REMEMBER THAT / MEANS "DIVIDED BY". In this example, t is the unknown number of teeth.

87/48 = t/64

The important thing here is that your pitch numbers have to either both be numerators (top numbers) or both be denominators (bottom numbers), likewise for the teeth number(s). The cool thing with ratios is that it doesn't matter which you choose for either (my ratio above spits out exactly the same answer if you invert the fractions and put the pitch numbers in the numerator).

To find t you will solve (87 X 64)/48.....remember to perform the operation in parentheses first...THEN divide.

This will give you the number of teeth in 64p on a spur gear that is the same diameter as the one you already have. This works because it is a strictly linear relationship. The other neat thing is that you will pretty much ALWAYS find there is a 64p equivalent to any of your 48p sizes (thought the inverse relationship doesn't seem to necessarily be true as we get down to very small 64p spur sizes). My guess is this is because the spur "blanks" are certain diameters and the teeth for either pitch are cut into those.