There is no magic weight or power level that dictates a two or three seed. It’s a grey area; there are a lot of variables. The biggest of these may be the engine torque curve. How flat the curve is, and how much torque is actually being produced, and on the other end, the mass or weight being pulled. Gear ratios in the transmission and I the differential are so important because gear ratio multiplies torque. In other words, a heavy car with a small engine must have a lot of gear to accelerate well. So saying that a big torque combination needs much less reduction or less gear. Comparing two speed to three speed, the two speed has very few rotating parts inside, so it takes very little power to run, equaling more power to the ground, if the two speed has enough first gear for the application. The three speed has twice the rotating parts inside, so it takes more power to run. In most situations, especially anything with big power and a limited tire, the two speed is best.
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