This gear was developed for the Blade 450 and 400, but I'm using it on my 200srx.

What I liked:
1. Made of metal
2. 10 teeth

What I didn't like:
1. Made of brass
2. Relatively course teeth. (Solution: Burnish with a powered brass brush and polishing compound. Usually it's not done on a gear, but read on...:
3. Wears down faster than hardened steel
4. Uneven teeth, and not exactly rounded which caused oscillations.

For use on my 200srx, I made the following three assumptions:
1. E-flite chose the 11 tooth gear for a typical altitude of 3,000 feet ASL;
2. The pitch of the stock rotor blade is 6 degrees;
3. Motor gain setting is 200 for optimized Work (W).

So why use a 10 tooth gear when the manufacturer recommends 11 teeth on the pinion? I live at sea level, and the air is "thicker" than at higher altitudes, and so a lower gear will help to cut through the air. What I get is better performance at take-off and mid-flight at the expense of top speed. All things equal, at the typical altitude of 3,000 feet ASL with the stock blade I get lower current draw and a longer flight time.

Nevertheless, I want to maintain the same level of Work as the stock 200srx at the typical altitude. The equation is pitch x (rotor diameter cubed) x (ration of new pinion versus 11T gear, namely 10/11) x 200. So, using a 10T pinion and an 8 degree pitched blade set at 165 will give me the same work output as the stock setup (except motor gain set for 200) at 3,000 feet. (the rotor diameter washes out since the blade lengths of both props are the same.) The result: Quit a bit more fun.