how to calc max rpm of dd hub in given wheel size off ground

so i can calculate what will match my desired riding speed?

i ask because i read this

Your efficiency will depend on whether your motor is suited to the task at hand. A motor that is too powerful waste a lot of energy spinning its own magnets/electromagnets past each other. A motor that is under-powered will waste energy if, when you run it at full throttle, it is still not able to reach its range of ideal operating speeds (at least ~50% of the speed that the motor turns with the wheel off the ground). Ideally you'll have a motor that will be running at about 80% of its maximum no-load (wheel off ground) speed most of the time.

also i am not good at math thank you

Neither am I. Which is why I love to oversimplify, and use rules of thumb, which are based on real world observations I make.

ALL calculations need to start with your weight. Lets assume your body weighs between 160 and 220 pounds. This is the typical weight of a mature man( or was anyway). My observations are my weight, 190 pounds, and I am on a bicycle, and not in a racing tuck.

Wheel size does not matter, for calculating max speeds, on flat ground. It DOES affect things though, like how much of your inputted power gets wasted into heat. (if you overload it) The load increases with grade,, so a reasonable load on the flat may be overloaded on a hill. The load increases with speed, due to wind resistance. Speed load is your rate of travel plus or minus the wind.

20 mph takes between 400 and 600 watts on "flat" ground, and less than 5 mph wind. Flat is defined as grades less than 1%. The variation is because you might be going down .5% and need only 400w. Wind can double the amount of power needed, even without being more than a breezy day.

25 mph takes more like 600-800w.

30 mph takes at least 1000w.

That's the simple chart.

If you want to really get into analyzing it,, then you need a program. the calcs are that complicated. Go to the website for Grin Technology, and use the simulator there. After all these years,, I'm not sure I really can read the charts. But I can see the obvious,, motor x on battery y, can only go z fast. The peak of the curves where they intersect is about how fast you will be able to go.

To calculate the Kv of a hub motor:

Mount it in a frame or vice, off the ground.

Connect to a controller, with a known voltage. (I use a 48V mean Well power supply, set to exactly 50.0V.)

Make sure the speed setting is programmed to 100% and no over spin, FOC, or other speed increase setting. I use a square wave Infineon controller, programmed specifically for 100% PWM. when I test my motors.

Put a piece of reflective tape on the rotor, spin up the motor with 100% throttle, and test the rpm with a digital optical tachometer.

http://www.ebay.com/itm/New-Mini-LCD-Display-Digital-Laser-Optical-Tachometer-Wide-Range-RPM-Measurer-YK-/152155409617?hash=item236d2b4cd1:g:54sAAOSwXeJXewkX

Motor Kv is RPM/source voltage: Example: 600 RPM at 50.0V = 12 Kv

It's also useful to note the current draw at No load, at the test voltage, as this will factor into the Km (Motor Constant) calculation, if you wish to know that as well.

* A word of caution: If you power your controller with a power supply as opposed to a battery, don't engage regen braking, as this could cause your power supply to blow from the reverse current.