I normally run a 21 or 22T on the 6.5t v3.0 motors indoors.
Turning the overheat protection is a must. If I leave the overheat protection on then the ESC will cut out. It seems very over-sensitve, but when I checked the ESC temps after a run with the program box all was still good.
Outdoors I've run upto a 23T pinion without any issue.
Make sure you are using the correct software and are not using the on-road 'Modify' software which has 'boost' on it. this is not really for off-road use.
For off road, you really need to be running 'Modify 0'.
Also check what timing board the motor has if its a v3.0, this will also make a lot of difference.
The latest versions of the software are on the website.
www.speedpassion.net
My settings are;
DDRS level: 3-5 (depends on track) normally I run 3.
Motor timing: Black board (middle on can)
ESC timing: 26.25 degrees
Overheat protection: Off