Four inches is the standard for cars and pickups on a properly compacted base. Go to 5 or 6 inches wherever heavier loads live: trailers, RVs, equipment, and the approach and gate areas they cross. Concrete strength of 3,000 to 4,000 psi is the common spec here.
More detail
Thickness follows the heaviest regular load, not the average one. A 4-inch slab over 4 to 6 inches of compacted base handles daily cars and pickups for decades. The places that deserve 5 to 6 inches are predictable: the apron where a gooseneck or camper swings in, the pad where a boat or tractor parks, the run to a shop door, and rural entrances that will someday see a concrete truck, a hay trailer, or a propane delivery. Thickening those areas at pour time costs a fraction of repairing a cracked 4-inch section later. Two companion specs matter as much as thickness. First, the base: thickness cannot rescue a slab poured on loose or wet-soft clay, which is why subgrade compaction and base depth appear as line items in every written scope here. Second, the joints: control joints belong at roughly 8 to 12 feet each way on a 4-inch slab (the working rule is 2 to 3 times the thickness, expressed in feet), cut to a quarter of the slab depth, so shrinkage cracks form inside the joints instead of across the surface. Concrete in the 3,000 to 4,000 psi range, reinforced and jointed correctly over a real base, is what a North Texas driveway spec should say. If a bid lists thickness alone, the important half of the spec is missing.