If you do not keep the overall diameter of your tire the same (wheel diameter + sidewall height) you will be altering your final gear ratio which will make your speedometer incorrect, alter your fuel mileage (better or worse depends on driving behavior and altered size), impacts your acceleration, what speed you can reach in which gears,
your effective power band, etc..
Though the literal
power band covers most of the operating RPM range, particularly in first gear (as there is no lower gear to shift down to, and no «flat spot» in which the engine does not produce any
power), the
effective band changes in each gear, becoming the range limited at the upper end by either the limiter, or a point roughly located between peak
power and the redline where
power drops off, and at the lower end the engine's idling speed.