MrMel Headspeed Calculator as seen online you can now get the Android version.
It gives you a good estimate how to gear your Electric Radio Controlled Helicopter.
It automatically fetching list of helicopters from MrMel's site so it's always updated with the updated list.
It will allow you to save preset (models) with your information which you can recall at any time.
It will also save last data used so you always come back with same info.
It can both give you estimated head speed or if you choose so it can give a suggestion of what pinion to use.
Do note that all values are estimated, true head speed may vary.