Q. Why must truck tyres be so large?

Answer

Actually one can also ask why trucks and buses have multiple tyres on same axle or why are trucks or buses multi axle. We see trucks with 14 wheels 18 wheels and even more and each wheel is quite large and wide. The answer is actually very simple. The idea is to reduce the pressure on the tyres and the road. Remember in our lessons of Physics we defined pressure as the ratio of force per unit area. Given the load on the truck the force is quite huge. Therefore by having wide truck tyres we increase the surface area to reduce the pressure. The truck and motorbus have bigger tyres in accordance with the adage "greater area less pressure" which states that as area increases pressure decreases and as a result less force is required to control the vehicle.