Count number of rotation of a rotating disc
Move the magnet and sensor in towards the centre of the disk. Speed will be slower and the magnet will be larger relative to the circumference at that radius.
The alternative is to put multiple magnets on the edge all touching each other and all in the same polarity. To do this find out the highest RPM you can reliably detect with the current setup. Divide that number into the max RPM you want to measure and you need that number of magnets.