welcome to CRF's Only and congrats on your new CRF250L. the 13T is one of the most popular mods to the CRF. it's why we include it in our performance packages. you can also use our RPM/Speed/Gearing calculator to see what this change will do for all three:
http://crfsonly.com/howto/gearing/motor ... ulator.php
again, welcome!
ken