I recently upgraded my 1/18 truck to a lipo and brushless setup. And it ran great, for about 5 seconds. The new pinion gear shredded the stock spur gear.

I'm pretty sure it was because the teeth on the pinion are smaller than the spur.

So I'm pretty confused how to pick a new spur. Do I have to get the pitch's the same? The new pinion doesn't have a pitch measurement, but is .4 module. is there a way to convert? Also, how do you know if the spur will fit your setup? All the spur gears i'm finding have different sized holes in the middle. Does each one come with it's own fittings?

Yes, the pitch must be the same. If either of them is different, they will not mesh properly and you will get what just happened.

What kind of truck do you have? Any setup information you provide will help someone get you the right info.

It's a Losi Mini Desert Truck. And the new pinion gear is an E-flight Pinion Gear, 14T 0.4 Module 2mm

Okay so I figured out that my pinion gear is a 63.5 or 64 pitch. I want to keep the same gear ratio that I had. My old spur was a 60t. The lowest tooth count i can find with 64pitch is around 70 or so.

Is a 60t 64p spur gear very rare?

You need to double check the manual....Losi sells spur gears (LOSB1068) and pinion sets (LOSB1260-1263). These should get you going. Just need to make sure that these pinions will fit the shaft of your motor.

Parts explosion:
http://losi.com/ProdInfo/Files/LOSB0202explodedview.pdf
Option parts:
http://losi.com/ProdInfo/Files/LOSB0202optionparts.pdf