A 13t? Dude what are you smokin'!?
Unless your running huge tires like Pro-line Masher 2000's that pinion is way to small.
For gearing you should first of all consult your trucks manual which recommends a 18 tooth pinion gear with the stock 87 tooth spur. I would start with a 17 tooth pinion and see how that works first off, go from there, if your running a really short track you may need to go down to a 16 tooth, if your running a really large track you will probably need to go up to a 18 or 19 tooth. If your motor is overheating in a 5 minute race that means your pinion gear is to large. There are alot of other factors that affect gearing as well, but the temperature of the motor is what you want to worry about. I find if my motor is getting above 160 degrees then its getting a bit to hot for me.