Originally Posted by
angrymelon
But a 72T 64 pitch spur is much smaller than a 72T 48 pitch spur is it not? Because of that, with a 72T 64 pitch spur I could fit a much larger pinion than with a 72T 48 pitch spur. Granted you can run a small 48 pitch spur like a 52T, but if memory serves, those were harder to come by, and you don't get the potential benefits of the smaller gear pitch as well.
That's why you use a 54 or 55T 48p spur to replace a 72T 64p spur. Take your 64 pitch gear, divide the tooth count by 64 then multiply by 48, and you get your equivalent sized 48 pitch gear. You never just look for identical tooth counts when changing pitch.
I have a 55T 48p Robinson Racing spur in my TC3. When I fit my 43T pinion, I get down to a 3.2 ratio.
Regarding the benefits of smaller pitched gears, sure. That's a valid reason. "48p won't fit" is not.