Where i live, 80% of all vehicles are passenger vehicles. I'm not sure that the extra wheels on semis would make up for that difference, especially with the slow increase in size of passenger vehicles.
20% * 18 = 3.6 vs 80% * 4 = 3.2, so barring some 3rd category semi’s would have more tires. They also have a lot more weight on each of those tires.
Semi tires are hard, long-lasting compounds relative to soft consumer tires with deep treads and soft rubber.
No, it scales at the fourth power of the axle weight.
So more and much larger tires and fairly similar lifespan = they liked make up a significant majority of tire pollution.