There is one question that I am stuck on at the moment and that is motor voltage. Almost everywhere I look I read "more voltage is more speed", which is true. But doesn't required power also come in to play?

For example, according to some calculations and reference builds on the web I came to the conclusion that I need about of 6.2 kW to get my bike to 90km/h. With the circumference of my wheels of 2 meters this comes to 750 wheel rpm. Let's also say I use a 50t rear sprocket.

Now I've been looking at the ME-1004 motor, and it has a nominal rpm of about 3100 (loaded) at it's rated voltage of 48 Volts I assume. 3100/750= 4.13, so if I use a 12t-13t front sprocket, won't this motor give me the required speed? If so, what would be the advantage of getting a motor with a higher voltage rating? Will it be more efficient because it runs at a lower current for the given output power?

I may be overlooking something here since this is all new to all of this, so any input is appreciated.