When race is on, the only thing that matters is concentration and thinking ahead. So you can’t keep track what gear is currently on. It is better to have this information in the field of view.
Recursive Development Labs have built reliable gearshift indicator. They used AVR micro with Arduino bootloader. Most challenging part here is to read gear data from mechanical gearshift. The room was enough to place small potentiometer which is exposed to vibrations that lead to unreliable readings over time. So they came up with dedicated calibration routine that allows to fine-tune sensing by saving calibration data to EEPROM. The output of this device is a single seven segment LED indicator which displays current gear shifted.