Here's a quick synopsis of gearing for different motors:
Higher-turn (less powerful) motors are usually geared to keep them near their peak horsepower for most of the time during the lap, just the same as for full-scale racing cars. ("Higher-turn" motors would likely include 17.5 on 1s LiPo, and 25.5 on 2s LiPo.) The horsepower peak on all permanent magnet DC motors occurs at 1/2 of the free-running RPM (provided the battery and speed control do not have significant losses). If you have the equipment to measure the length of your racing ("run") line on the track, the RPM of the motor, and know your expected lap time, you can calculate the correct rollout as follows:
Rollout (inches/revolution) =
[1 / free_running_motor_speed (revolutions/minute)] x
[run_line (feet)] x
[12 (inches/foot)] x
[60 (seconds/minute)] x
[1 / lap_time(seconds)]
(You'll probably want to change the units to metric since you are in Germany.)
If these values are unknown, start with a conservative recommendation from a fellow racer who is familiar with the track.
More powerful motors are usually thermally-limited, meaning they would overheat if geared as described above, so they must be geared so their average speed is above the peak power speed. Select a rollout to keep the temperature in the range recommended by the motor manufacturer (usually 140F to 160F maximum). Again, start with a conservative recommendation from a fellow racer who is familiar with the track. The actual rollout you need will depend on your driving style, which is reflected in how many mAh you pull from the battery during a race. Smoother, more experienced drivers get around the track using less throttle than others, and can use a higher rollout without overheating the motor.
When race results are re-calculated using the IOF (Index Of Fun), I always win.
1993 ROAR 1/8 Pan National Champion