That is to prevent over-revving the engine in the lower gears of a big rig with a Brownie and/or a 2-speed rear end. The really low ratios needed for pulling heavy loads needed an engine with lots of torque, not necessarily a bunch of high-RPM horsepower. That cone was probably designed to keep the engine to a safe top speed in the lower gears.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!