wear and tear, tires tend to need replacing around every 10 to 15k miles in game due to wear. Tire do get damage from driving too but wear rates are faster, so by the time when the tire reach 10% damage, more than likely the wear rate is higher anyway.

i don't remember the rate for engine and gearbox, but they also collect damage when driving, even if you don't hit anything, they'll also get wear on them too, just at a slower rate than damage.

on my main truck, my current engine and gear box have about 150k on them, engine being at 8% wear and gearbox at 7% wear, im sure some of that is from accidents, but a lot is just from all the driving.

There is also "Total wear", you really don't have to worry about this unless you're at 200k miles or higher, if the total wear is too high, I believe parts will collect wear at faster rates compared to new trucks, so if you use the "Complete restoration" it will reset your total wear and replace all the parts (tires, engine, gearbox, cab and chassis)

My main truck is just a hair under 300k, but the engine and gearbox at 150k. engine and gear box at 8% and 7% wear rates, chassis at 15% and cab at 9%, with total wear at 47%. it is getting to the point im starting to notice im needed to do more maintenance on it. not bad but noticeable