I am in need of some advice. My son is turning 13. He rides a crupi expert xl and I can not match up the gear charts for the equipment he has. What are your suggestions for the correct gears.

13 expert N.A.G. #9 cruisier.
Crupi expert xl. 20"
1/8" rim.
1 3/8" tire
170mm crank
Height 5'0"
Weight 100#

Currently he is running a 14 - 41 and is spinning.

What recommendations can I get?

If he can spin a 41-14 keep him on it. But that is a about a 60 (41/14*20.5). The fastest kids in his class are probably running around a 54.6 on 1.50's. That would be about a 40-15 on 1 3/8's with a 20.5" diameter.

Generally, you want to be on top of your spin at the end of the first straight. So, I say forget the ratios and just watch and see what he can push and get up to speed without running out of gear. It's pretty basic, but very easy to get caught up with over engineering the numbers.

I know an 11x that runs around 50-51 gear inches and wins nationals. I know a 12x that runs close to 60 and wins nationals. My sons 12x and runs around 54-55. I learned a long time ago, you pick a gear that suits his riding style and leave it the h@#l alone! 170 cranks seems long for 5' to me though.

Skull summed it up best!

that stuck out to me as well....back him down to 165 maybe