DP stands for diametric pitch, and is the only kind of pitch used when describing RC gears so "DP" and "P" and "pitch" are interchangeable terms. (Worm gears and rack gears are the types of gears that have non-diametric types of pitch.)
Diametric pitch is how many teeth per inch-diameter (hence the name) of a pinion or spur gear. A 32P gear, for instance, has 32 teeth for every inch the gear is in diameter:
- a 1-inch diameter gear would have 32 teeth
- a 1.25-inch diameter gear would have 40 teeth
- a 2-inch diameter gear would have 64 teeth
Higher pitches (48P, 64P) have more, finer teeth for a given gear diameter than lower pitch (32P) which are larger and coarser.
"Pitch" is the standard/imperial term. "Module" is the metric term and is calculated slightly differently. Module is the millimetres-of-diameter per tooth, rather than teeth-per-diameter used with Pitch.
0.8-module (sometimes abbreviated to mod-0.8, Module 0.8, or 0.8M) means the tooth is 0.8mm in diameter for each tooth, so a 100-tooth gear would be 80mm in diameter.
With Module, lower numbers (0.6, 0.4) means lower diameter for a given tooth count (finer teeth) and higher numbers (0.8, 1.0) mean larger diameter for a given tooth count (coarser teeth).
Bigger teeth (lower pitch or higher module) are stronger, but noisier and since you have to move in 1-tooth increments when changing gear ratio, the available ratios can't be tuned as finely. Finer teeth are quieter and more finely-adjustable, but aren't as strong.
0.8M and 32P are practically identical in tooth size, and these are the only compatible sizes between metric and standard (in common use with RCs). All other modules/pitches commonly used in RC are not compatible with any other. If you come across a "metric 48P" then it is NOT compatible with a true standard 48P.
Common pitch per vehicle type:
- 1/8th electric and nitro: 1.0M
- 1/10 nitro: 1.0Module or 32P/0.8M
- 1/10 electric 2WD offroad: Usually 48P. High-power brushless conversions often convert to 0.8M/32P to handle the extra torque
- 1/10 electric 4WD offroad: Often 0.8M/32P
- 1/10 electric onroad: Often 48P, but 0.6M for many Tamiya and some others. Sometimes 0.4M or 64P for higher-end cars.
- 1/16 and 1/18: 48P or 0.4M. I'm not so sure about these smaller cars, though.
Some helis use uncommon pitches like 0.7M or 0.5M.