Here’s a low-tech solution (if your bike has a gear indicator output like Phalanx said.) Use any common anode display, leaving the decimal point unconnected. The diodes are 1N4148 (or equivalent) and use 820 ohm resistors to keep the current to a safe level to avoid damaging the display. Connect the appropriate line (N-5) to ground to get that number displayed. This circuit will only work with one of those lines grounded at a time, otherwise you’ll get weird characters in your display. If you need more numbers, just add some more diodes to light up the segments you want. I left the pin numbers off the display because I have no idea which display you might want to use. Also, here’s a link which might give you some ideas. These folks use a more complex, higher-tech approach.
**broken link removed**