Depends on the season...
Here in South Caronlina, locally known as God's Country, the prefered truck rifle varies due to seasonal concerns.
Since Deer season runs from Aug. 15th to Jan. 1, during that period, the favorite truck rifle is the current deer rifle[M70 Win. in .308,30/06, or 300winmag]. This is because you never know when you might get off work early enough to make it into a treestand before dark
After deer season, the truck gun is usually a .22, .22 mag, or if I'm feeling frisky, my AR. This time of year, the truck gun is there mainly because you might need to dispatch a feral dog/cat/hog that is causing problems....or you might just get of work in time to go plinking. The rifle is mainly in the truck this time of year because....well....it just doesn't feel right not having one in the truck.
Of course the social reasons that the "truck rifle" may be called for are always in mind, but any of the above mentioned rifles, plus my CCW .45acp, should be enough.
Oh yeah, during the month of April, the truck rifle is replaced with the truck shotgun[ Remington 870 turkey express, 12ga, 3" mag], becasue you never knowwhen you might get off work early[or decide to go in late] and go turkey hunting.