Transmission - For extreme off road go carts the optimum choice is a transmission and transfer case for four wheels drive if possible. If this is out of your price range or capability to build there are other options.

Centrifugal Clutch - The most simple is a centrifugal clutch. These are most common on single wheel drive go carts or fixed axle go carts. However with the correct axle it can be used with a pose rear. The trick is to have the sprocket gear ratio to what you want. With a centrifugal clutch there is only one speed.

Torque Converter - The next best thing to a transmission is a torque converter. The torque converter is like an automatic transmission as the engine RPMs increase so does the gear ratio. With a torque converter, the process is 100% automatic the speed of the cart controls the gear ratio. The problem for extreme off road go carts is that a torque converter must use a belt to drive the gears because it is required to stretch a chain will not stretch. So, off road in the rain or through water can cause the belt to slip.