mbs wrote:yes! i'm currently looking for a 20 inch diameter, and operated at 48v. i did already look at their site before, it seems that 9c 2807 would already fit my ebike project but i'm still in search for a better motor(efficiency curve) right now.
OK, we're slowly getting there................
Again, what's the total weight and cruising speed that you're looking for?
As I tried to explain above, motor efficiency is very highly dependent on load. A motor winding choice that gives good efficiency at one rpm and load will likely give poorer efficiency at another speed and load, it's an inherent property of all permanent magnet motors.
For example, say you decide on a 9C 2807, as you suggest. This will be around 84% efficient when running at about 22 mph on 48V with a 40A current limit controller with decent FETs. If you intend to cruise at around 22mph then this would be a reasonable choice. If you cruise at around 15mph (about 60% throttle), then the efficiency comes down to around 82%, which is still reasonably good.
If you want more speed, then look at the lower winding count motors. For example, sticking with the 9C range, a 2805 could give you around 84 to 85% efficiency at around 30mph, but as with the 2807 efficiency will drop off at lower speeds. For example, at 15mph the efficiency would be down to about 80% (around 45% throttle).
As said before, there is no "best" motor, only the best compromise for the particular way you want to ride.
Please ask questions on the forum, rather than by PM, as it helps others and you'll get a better range of answers.