The drive gear for a third gen firebird with an electronic 140mph speedometer 1986-92 for firebirds, possibly 90-92 for camaros. This is the gear that falls apart, keeping the odometer from working. It can be quite difficult to replace. I ended up breaking the peg off and soldering it back because the shaft doesn't have a removable clip. You must remove the speedometer needle and completely tear down the assembly to replace. Make sure you mark the needle resting location on the counter weight below the gauge face. If you aren't comfortable advanced diy I wouldn't recommend trying this.
The center hole is 3mm but may be smaller in the prints. A 1/8 inch drill bit should be able to enlarge the hole to let it fit smoothly
An alternative method for replacement would be to drill the peg out and tap the hole and replace the peg with a screw of the appropriate size 3mm or 1/8th OD. The threads shouldn't interfer with the rotation too badly. I haven't tried this method but it should allow for replacement with less tear down.