Speedo drive gear demolished!

So I posted on here a couple weeks ago about having my 4 speed rebuilt. I got my car back last night, test drove it, and it's a lot better, however I still have a vibration somewhere. The new clutch felt amazing once I removed my over center spring, and the car actually feels faster.

So I decided to take a little evening cruise last night after getting it back, I didn't hammer on the car just a little spirited driving. I went about 15 miles, and come up to a stoplight. When I go to take off I notice the speedometer isn't working. I checked the cable, it was good so I pulled the drive gear out, and it was absolutely demolished.

I'm going to order a replacement, how can I prevent this from happening again? What caused it to happen this time? Any tricks on reinstalling it?

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