A sample list of gear ratio shown below
The factory suggested gear ratio sample shown below
Example of checking the gear ratio by relevant motor gear & spur gear
1. Check the factory suggested gear ratio first
2. If the motor factory suggested gear ratio is 7.085
3. Look at the gear ratio list
4. Therefore the spur gear 93T & motor gear 28T are needed
So easy 
Source: www.rcmart.com
