Well I'm not sure..... I thought I had an understanding for it.... But u mad me feel dumbIs the gearing ratio above intended to be for the pinion/spur ratio only or is it also supposed to take into account the rest of the transmission?
UPDATE: I looked it up and they intend for this to be used with the final ratio. (http://www.area52.cc/how-tos/403-pick-the-right-brushless-motor.html)
Now for some math...
Let's use the stock gearing on the Granite MEGA as an example since it has the shortest gearing as stock.
Drive Ratio: 87/15 = 5.8:1
Layshaft Gear: 15T
Internal Ratio: 39/15 = 2.6:1
Final Ratio: 5.8 * 2.6 = 15:08:1
Now the Granite Mega comes with a 15T 540 can motor... which falls in between two choices on the chart above and the 15:1 ratio of the Granite MEGA isn't even close to the ratios they recommend for the 17.5T or 13.5T motors.
To get to the 9.0:1 ratio that they recommend for a 13.5T 540 can motor for off-road practice you would need a 25T pinion with the stock 87T spur and the same internal transmission ratio of the Granite MEGA.
9.0 recommended ratio / 2.6 internal ratio = ~3.46 ... 87T spur / 3.46 = ~25T
I'm guessing that a 25T pinion on the Granite MEGA is going to cook your motor or ESC pretty quickly on the 2S LiPo they recommend. Does anyone here want to try it to prove the theory?
Bottom line... use some caution with the recommendations in the chart. Perhaps the Arrma's weigh a bit more or use larger tire diameters than what the chart publishers have in mind and that affects the calculations somewhat.
I would appreciate it if someone could please check my math and thinking to be sure I'm on solid ground here.