More power does not give proportionally higher maximum speed but it does mean more speed going uphill. Just as with a car’s motor in any gear there is a speed range where the motor performs most efficiently while at much lower or higher speeds the motor will either struggle or max out. Electric motors have various performances,
as fuelled motors do.
As you travel the three main forces you have to overcome are wind resistance, rolling resistance and gravity. The least is rolling resistance. The effect is like you are constantly riding up a gentle slope. If you roll down a gentle slope at a steady speed (not gaining) then that slope is the rolling plus wind resistance at that speed.
Bearings condition, brakes rubbing and low tyre pressure affect the rolling.
Wind resistance goes up rapidly at increased speed. 50% faster causes about twice the wind resistance. As does sitting upright. That’s why racing riders tuck down low.
Power needed for climbing hills is of course proportional to the slope.
Neglecting other resisting forces you can work out approximately how much power you need for a hill :
Watts = weight (kg) x speed (M/sec) x 9.81 x gradient (%)
or 100kg at 20kmh at 5% (maybe that’s 4% hill + 1% w+r) = 272W
100kg at 28kmh at 7% (maybe that’s 5% hill + 2% w+r) = 534W
That is approximately what you can expect from 250W and 500W motors.
Note that wheel size and torque are not in the equation.
There is some confusion (and people trying to confuse) about power and torque.
It is power that gets you up a hill, not torque.
Smaller wheels and/or a motor driving the chain, along with lower gears may allow a motor to work more efficiently and so provide more power.

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