Fresh Start - Motor & Esc

[DJ's Dad]

Just had an expensive club night! First time out in public and we've stuffed the speedo and the motor's looking none to healthy either.
Daniel managed 3 heats, with the motor SP v3 7.5 getting v.hot. In the 4th heat after the 4 minute call it got too hot and took out the speedo - SP LPF 2.0

We'd had 5 or 6 runs at the park and the motor was hot but nothing blew.

Gearing was 21/87

Had I over geared it?

I'd bought him the 7.5 to tame things down a little, only been racing 9 months.

I don't want to make the same mistakes again.

Thinking SP 6.5 or 5.5 with either LRP SXX TC V2 or SP GT Pro both with fans.

How do I work out the gearing limits?

Thanks