"My suspicion is that back in the 40's and 50's just as today, trucks that stayed on the road with infrequent stops and starts racked up a lot of miles with minimal wear. "
In the 40s and 50s (and most of the 60s) there were no Interstate highways. Most small trucks did a lot of start/stop (up-shifting and down-shifting) driving on winding roads and "crowned" roads. There was significant wear and tear on engines, drive-trains, brakes, wheel bearings and suspensions (compare to average driving conditions/roads today).
