How can the tires do that to your gearbox? I can imagine that you develop more torque on the diffs, but flatten the bearings on your gearbox???
I can also imagine that the gearboxes have been "beefed up" due to the much larger engines in the 463s.
I can also imagine that the gearboxes have been "beefed up" due to the much larger engines in the 463s.