Long before General Motors used the name "Jimmy" for their SUV, it was taken to mean the GMC tractor which pulled semi-trailers over the road. White was another maker of tractors. White cabs had characteristic lines which made them stand out on the roadway. A White cabover wouldn't be mistaken for anything else.

Don't know about Georgia overdrive, but out here in California, High Sierra Overdrive means throwing the transmission into neutral and coasting (often hell bent) downhill. No doubt Spaw will have an answer for Georgia overdrive.

Dave