The air pressure in a tire, multiplied by the area of the patch of rubber in contact with the road equals the force down against the road (at that particular tire). By Newton's 3rd Law, the road exerts an equal upward force on the tire.
A large tire has a larger patch of rubber in contact with the road. Therefore, the pressure does not need to be as great to allow the formula
#"air pressure in a tire" times "the area of the patch" = "force"#
to yield the necessary force to support the truck.
Sorry about the American English. I have trouble getting my fingers to type tyre. Hope this helps,
Hi, in a former life I worked for a large tyre manufacturer (mainly m’sport, but did enough R&D to add something here.)
The load rating on a tyre is the two/three numbers after the tyre dimensions followed by a letter e.g. 165/60R14 74 H
This tells you that the sidewall to sidewall distance (approx, the contact patch Steve mentioned) is designed with a width of 165mm (actual figures vary depending on speed, load, wear etc.) The height of the sidewall of the tyre is 60% of the “tread width” above, the rim size is 14” (inches) in diameter and finally the load rating, 74 followed by the speed rating, H (ignore that for the moment, it’s largely theoretical anyway.)
OK, now we can read tyre markings (or the basics anyway) we can explain a bit about more load ratings - they vary from the low 60’s to 140 according to the ETRTO Standards Manual, but commonly range just 70-110. The scale is rather arbitrary, but approximately linear and is different for different wheel configurations (twin rears on trucks/light trucks) and for P-metric tyres.