http://www.horizonhobby.com/Products...%20Accessories
has a good description of what pinions are for what. Keep in mind this is for the CP, not the pro
8T pinion - This motor and pinion deliver extra-long flight times when using 3-cell LiPo packs and flat bottom main blades.
9T pinion - Same motor and pinion included with the Aerobatic Enhancement Kit. Recommended for use with 3-cell Li-Po packs.
10T pinion - Stock motor/pinion. Best for 8 cell NiMH and flat bottom blades.
11t pinion - This motor and pinion are for use with 2-cell 730-1320mAh Li-Po packs and flat bottom main blades.
I think on the pro the stock is 9T, or alteast that is what mine has
The smaller the T number on the pinion, the lower the head speed if everything else stays the same, and visa versa