There's probably way too much for anyone to soak up and understand within a day of reading forum posts, or even reading a good book on motors and gearing. It really takes time, experience, failures and above all patience. However, here is my attempt to provide several thoughts around this subject that hopefully will help. This is off the top of my head, in no particular order and certainly not thorough. Mindless dribble...
'Taller', 'larger', 'bigger', 'more' gear(ing) can be equated to pinion sizes or rollout numbers, but are opposite of spur sizes and drive ratios. So if you currently use an 84t spur and 22t pinion and are recommended to go to a taller gearing, you would switch to a larger 23t pinion which also results in a larger rollout number. Or you could switch to a smaller 80t spur for close to the same result.
In touring car racing with rubber tires the diameter doesn't change. Therefore most drivers here use the Final Drive Ratio (FDR) numbers to compare and select the right spur/pinion sizes (referred to as gear combo). When using foam tires, where their diameter is constantly getting smaller, most drivers use Rollout numbers to compare and select the right gear combo. Keep in mind that some drivers use metric numbers (most XRAY drivers do, as the company is based in Europe and all documentation/tools are metric) and some use standard numbers.
Either way, correct gearing depends on a host of variables, that include the following:
* Motor model/setup/parts(arm/can/endbell) - There is normally a 'range' of good gear combo's for each model (i.e. co27, Monster, or Chameleon 2). With two motors of the same model that have differing brush, spring and parts, you could end up with one that has relatively high rpm and low torque and the other that has relatively high torque and low rpm. The high rpm motors best gear combo may be an 84/20 for a track with tight corners and an 84/21 for a more open fast track. Whereas, the high torque motors best gear combo may be an 84/22 for a track with tight corners and an 84/23 for a more open fast track. Motors with high torque require larger pinion sizes (or smaller spurs) and other motors, of the same model
, with higher rpm require smaller pinion sizes (or larger spurs). The more torque a motor has, the easier it is for the motor to accelerate the car with a taller gearing (larger pinion/smaller spur). Even if you use the same motor, but change to different brushes/spring tensions, the best gear combo will normally change as well, seeing as the motors torque/rpm characteristics will change. Another thing to note is that certain motor models run better at higher temperatures (p2k2) than others (Binary 2) as well.
* Track layout - A layout that has a lot of corners and/or few or small straights typically requires smaller gearing (smaller pinion or larger spur). And a layout with few corners and/or more than one long straight typically requires taller gear combos. So if you have already figured out
that your current motor's best gear combo is an 84/21 for a tight track layout, then you would probably want to change to a taller 84/22 gear combo to start off with for a faster more open track layout. However, you really have to make sure you pay attention to the turning radius of corners on a track and not just the number of corners and number/length of straights. Our current local tracks layout would seem to be a fast track, requiring taller gearing, seeing as it has four straights. However, we've found that the straights make the motors run cooler overall, but that the four tight 180 degree corners actually wreak havoc on the comm temperature. So the cooling of the long straights actually masks how hard the comm is working, and getting hot, to get the car out of the 180d corners. A good motor temp for a p2k2 used to be 195F for our previous layout which had much less straightaway and more cornering. A good motor temp with the current layout is a good bit less at 180F coming off the track after a full 5 min run.
* Environmental temperature - Not rocket science. Cooler air termperatures keep motors and other things cooler.
* Speed controllers (ESC) - The better the speed controller is, the more efficient it will be and the cooler the motors will run relatively speaking. ESC setup, drive/braking frequency (a.k.a. punch control, etc.), plays a role as well.
* Car setup - If you use the same motor between two cars of the same model, but with different setups, the best gear combo will normally change.
* Drivers style - (throttle control) How early and quickly you pull the trigger. (steering input & driving line) How smooth and how long you turn the steering wheel.
Now the best thing to do for someone without much experience is to ask for help from local experienced drivers. Its much easier to explain and understand while actually having a verbal conversation with someone.