Tamiya 540 25T Motor with Grasshopper

Hi brothers,  I have installed the sport tune 25T 540 motor into my grasshopper, however, I have 2 questions.

1. When reversing the motor, a lot of sparks and the motor is very noisy (there is only little spark when forwarding)

2. According to the motor spec, it runs on Gear Ratio 6.5 - 7.5, but I dont know the gear ratio of my grasshopper, can any brother help?

Thank you.