system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do tractor trailer trucks need 18 large wheels, but very large planes only need 6 small ones?
gpt
Trucks drive a lot of miles and weigh a whole lot, so their weight needs to be distributed.  Otherwise they're causing a lot of wear on the pavement.  Virtually all wear and tear on highways is caused by trucks, so spreading out the load is very important. Planes only "drive" a short distance on pavement, so there's less infrastructure being damaged by having the load more concentrated.  

Additionally, putting more tires on a truck is fairly cheap (you have a nice rigid frame to bolt the axles to) and can make the truck safer (losing one tire out of 18 is relatively minor; losing one out of 6 is a bigger deal).  By comparison, putting more tires on a plane means that you increase the weight by quite a bit.  Losing a tire on landing is pretty inconvenient, but there aren't other planes out there that you're going to hit if you get a flat, so the safety issue is less present.

Then you look at the life of tires on trucks versus planes.  A truck is looking to drive thousands of miles before they have to replace their tires, so you want to have lots of tires to reduce the wear and spread the load.  A plane would ideally like to do a couple hundred landings on a set of tires, which is likely under 1000 miles all told; some planes may only do 10 landings on a set of tires (looking at you, SR-71).  It's OK to have to replace tires all the time on a plane since carrying around more tires would cost even more.

Finally, note that while planes are larger than trucks they aren't *that* much heavier.  A truck may be 100,000 pounds, all told, while a 737 (6-wheeled aircraft) maxes out at about 150,000.  Meanwhile, *very* large planes like the 747 have far more wheels—18 for that model.  The C-5 Galaxy has 28 wheels.  The An-225 has a whopping 32 wheels.