Gears:
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All V5 gears are 24DP, which means there are 24 teeth per inch of pitch diameter. So a 12T gear would have a pitch diameter of 1/2" (12T / 24DP). The 24T gear would have a pitch diameter of 1". The pitch diameter of all the V5 gears are:

12T = 0.5" Pitch Diameter
24T = 1.0" PD
36T = 1.5" PD
48T = 2.0" PD
60T = 2.5" PD
72T = 3.0" PD
84T = 3.5" PD

To work with the existing holes in Vex metal, the radii (not the diameters) of two gears have to add to a multiple of 0.5".
12T + 36T radii add to 1.0", or two holes apart.
12T + 24T gives you 0.75", which will be halfway between two holes.

Gear ratio is the relationship between the circumference of the gear on the driven part and the circumference of the gear on the driving part (motor). When several gears are meshing together, they form what's called a gear chain. The ratio is only calculated from the first gear (the driven gear) and the last gear in the chain, which is the driving gear, the one attached to the motor. The ratio represents the increase in torque (and the proportional decrease in velocity)

Possible combos:

12:36 | 1:3 |  66:200:600  | 200:600:1200 
12:60 | 1:5 |  40:200:1000 | 120:600:3000
12:84 | 1:7 |  28:200:1400 |  84:600:4200
36:60 | 3:5 | 120:200:333  | 360:600:1000
36:84 | 3:7 |  85:200:466  | 255:600:1398
60:84 | 5:7 | 142:200:280  | 426:600:840
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24:48 | 1:2 | 100:200:400  | 300:600:1200
24:72 | 1:3 |  66:200:600  | 200:600:1800
48:72 | 2:3 | 133:200:300  | 400:600:900
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offset:
36:72 | 1:2 | 100:200:400  | 300:600:1200
48:84 | 4:7 | 114:200:350  | 342:600:1050
36:48 | 3:4 | 150:200:266  | 450:600:800

https://www.vexforum.com/t/catalogue-of-drive-gearings/109498