change gear ratios

Hi, how can you change gear ratios.

Go to your local Hobby shop, get either a pinion gear or spur gear with a different number of teeth. However you need to check the pitch of the gears to insure a correct fit.

How come some transmissions say that they are fully adjustable???

Trannies on gases are all different. This is where it can involve knowledge of how RC's work, and mechanical engineering.