Question: why are semis called 18 wheelers?
Answer: Since one axle has two hubs, an axle with dual-tires counts a total of four tires. So in the colloquial term “18-wheeler”, the word “wheel” here means “wheel rim”. An 18-wheeler has a total of (2+4+4)+(4+4)=18 wheel rims, and hence 18 tires.