Size of the pinion will depend on your track where you plan to run. 7.5 turn motor is crazy fast. I have eb410 with 13.5 motor, and believe me it's balistik. For example there is 2 tracks in my area, one is hobby town that is tiny and you don't need a lot of speed, so small pinion. The other is thunder dome that is decent size, and you need significantly higher speed and bigger pinion. Like I said, 7.5 motor is crazy fast, and on top of that you are new to racing, I would start with smallest pinion, 17 tooth. And then if track is large and once you get some experience you can easily gear it up
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