This property sets how the engine speed affects gearing and automatic down-shifting. It is set for land vehicles and planes in Physics.con. For example, 0.5 means your car gear goes down to the next lower gear when half engine speed or less is reached.
Values used in BF: 0.3 for planes, 0.4 for jeeps, 0.45 for other land vehicles.
See SetGearUp for more explanation about gears.
Source and more details