Imagine cars to be without gear ratios and all, and just a
engine connected to wheels:
If both at travelling at 100km/h, assuming a same car shape, the
air resistance/drag will be the same, and the power put into the
car will be to overcome this resistance (newton's first law of
inertia) to travel at const velocity.
But with gear ratios this complicate things up. Because If both of
them are having the same gear ratio, and both are travelling at
100km/h, then the RPM must be the same because the distance moved
per unit time is the same, hence the amount of rotation by the
gears must be the same, and the gears being connected to the
engine, and hence the engine must be rotating at the same
speed.
