Check out Hondagrom.net and check the racing sections. There are a good amount of people that race their groms and some gokart tracks let peole take groms/z125/etc. on them.
My recommendation would be to go with what these guys run. Or something similar. The good thing is you'll be able to see reviews on what people have ran on the groms. Outside of that... I don't have much advice. I'm still running the stock tires.
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