Sjoe, i think there is going to many theories here.
Besides taking it to a professional mechanic, – i think it could possibly be the clutch plate.
Something is causing friction when you change gears, i would assume that when more revolutions are made this increases friction which in turn creates more heat. ultimately burning some form of rubber coated wire or seal.
Strange how you can be able to distinguish this smell from inside the cab at 100km/h?
are you able to notice this smell while driving at 100km/h, or when you come to a stop.
Also, has your problem occurred out of the blue? or was it fairly recent after a service?
I had a similar problem, but that was caused by my wife when she forgot to let the handbrake down! LOL :silly: