Here is a Circular Teeth Profile Gear Calculator Program.             
Number of Teeth :
Theeth Width: In pixel
X Offset value:
Y Offset value:

Best of my knowledge these calculations are OK, use it with your own risk.

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How to generate circular teeth gear in SketchUp
1. Copy the code block from the buttom of this page
2. Open a text editor something like Note Pad or Text Pad and paste the code in it.
3. Save the file with a name such as "test01.rb".
4. Open an empty SketchUp sketch
5. Open ruby consol under the Window menu.
6. Enter "load "c://test01.rb" in the command line.
7. Use push pull tool to give a thickness to the gear.
8. Below is what you will get.