As this mod strives to be as realistic as possible, it changes many of the damage system's variables. These changes, the thoughts behind them and the sources I used are detailed in this discussion post. If you have any comment regarding my methodology, the sources I used or know of better values, then leave a comment in this discussion or on the mod itself.

This post uses a lot of the terminology of my guide on the damage system and assumes the content of it to be known to the reader. If you haven't read it yet, you can find it here: The Damage System - Explained.

The game offers a lot of variables for modders to tweak the damage system. Sadly, no variable (at least known to me) exists to control the damage threshold at which the warning lights in the truck turn on and damage effects start to apply. The default for this is 15% of total damage on any core part of the truck. Due to this limitation, I had to work around this default threshold value when planning the wear of the truck parts. It would have been better for SCS to increase this threshold a lot more to let parts get to nearly 100%, which would have given a better feedback to the player that a part should now be replaced. Because at 15%, I feel like, a player might assume that the part still has a lot of life in it and does not understand why it already has to be replaced. This is an area where SCS could improve the damage system. But for the time being, I worked around this 15% threshold to be the time at which parts start having problems and should be replaced. Keep this in mind when playing with this mod.

Variables changed in damage_data.sii:

Let's start with the wheels of the truck and trailer. Wheels, according to an SCS developer [1], include the tires, rims, brakes and dampers of the truck. In order to determine the damage values for this part, one should take all of these parts into account, not only the tires itself. But since the prices for wheels in the game actually reflect the real prices of tires quite accurately, I think that wheels should only include the tires of the truck. Additionally, the source I used for the cost of repairs has the expenses for tires separated from other repair and maintenance costs. Therefore, I have included all the other parts of the wheels (such as brakes or dampers) in the calculation of the other parts of the truck.

Originally the wheels of the truck reached 100% of wear after only 31,000mi or 50,000km. However, according to my research, the tread of a tire can last about 175,000mi on the steers and up to 375,000mi on the drive tires, but the tire itself can last much longer. [2][3]
As a result, tires are often only retreaded rather than replaced when the tread is worn down. This can be done two or three times before the tire is too worn and needs to be completely replaced. In addition, tires are often balanced to last longer, so I tried to include that cost in the calculation as well.

In conclusion, this means that the tread on new tires will last an average of about 275,000mi across all axles (they can't be separately repaired in the game). Each retread reduces the possible mileage to about 90% of the previously driven mileage. Michelin, for example, offers a warranty on its tires for either 700,000mi or 3 retreads. [4]
Using these two pieces of information, I concluded that two retreads are feasible before the tires should be replaced. Over the lifetime of the tire, the average mileage at retreading is approximately 250,000mi.

The cost to retread all the tires on the truck is $2,000 ($200 per tire [5]). Alignment and balancing of all axles costs ~$450 [6][7]. Assuming the tires were aligned twice prior to retreading, the total costs add up to $2,900 per retread. This price is almost perfectly in line with the statements I found online that the cost of retreading is 1/2 the cost of new tires [8]. So ideally, tires should have 50% of damage after ~250,000mi, which results in 1.25e-6 damage/km.

Over the life of the tires, the amount of damage at the point of retreading will decrease, but the amount of wear on the tires will increase. At 750,000mi, the tires will have 15% wear and 45% damage. At this point, the player should replace them with new ones (similar to the other truck parts). Dividing the target percentage of 15% (0.15) by the desired mileage in kilometers (1,207,008km) gives 1.25e-7 wear/km.

Trailer tires tend to last longer than truck tires, at about 400,000mi [9]. Due to the longer tread life, they are retreaded only once before the warranty expires. Calculating with the same cost of 50% to repair them after an average of 380,000mi driven, results in 8.18e-7 damage/km. Wear will increase at a slightly slower rate than on the truck to match the reduced damage.

Unfortunately, it's not easy to give the player feedback on when to repair or replace tires because there are no indicators and the damage effect is quite small compared to the other parts of the truck. Because of this, I also tried to have them be at the end of their life at 15% of wear to be in line with the rest of the truck's parts and my recommendation to replace them at that point.

I excluded the wheels from the following calculations, because the source I used also separated them from other service costs. [10] To calculate the cost of repairing the core parts of the truck, I used the average annual repair and maintenance cost of a truck as a base, which can be as high as $15,000. [10] On average, a trucker drives around 100,000mi per year. [11] A truck should be serviced roughly every 50,000mi. [12]
To calculate the average cost of each service visit, I calculated the total repair and maintenance costs over the life of a truck, which is 19 years * $15,000 = $285,000. The life of the truck is calculated by dividing the 100% permanent wear mileage, explained later, by the annual miles driven, which results in approximately 19 years. The $15,000 annual repair cost mentioned above includes replacement costs, so these must be subtracted from that cost.
On average, a truck has one engine and two transmission replacement (based on the values explained below) in its lifetime, which costs an average of $50,000 + $25,000 = $75,000 in the game. So the actual service cost would be $285,000 - $75,000 = $210,000. Per year and 100,000mi, this comes out to ~$11,050. The cost of an average service visit would thus be $5,525.

Since there are no insurance costs natively in the game, I wanted to include them in the service costs as well. On average, U.S. truckers pay $6,500 annually for insurance. [10] Adding half of that to our service visit cost increases it to $8,775. This includes the truck and trailer. Considering the price of a truck vs. a trailer, I split the service cost, 2/3 on the truck and 1/3 on the trailer. A truck service therefore costs $5,850 and a trailer service $2,925.

To inform the player of the need to service the truck, I used the check engine light and visit service message on the truck's dashboard, as well as the DOT warnings. In order to trigger these indicators, a part must have 15% of total damage. I used the transmission to achieve this, as it's the least expensive part of the truck on average.

The transmission of a semi truck should last 500,000mi with proper care. [13] To calculate the wear rate, the transmission should be at 15% wear after 500,000mi, which results in 1.86e-7 wear/km. By the time a service is due (after 50,000mi), wear will have increased by ~1.5%. To trigger the service indicators, an additional 13.5% of damage is required. This corresponds to 1.68e-6 damage/km.
Obviously, this only applies to the first service visit. Successive service visits will occur after fewer and fewer miles until the part requires a complete replacement. The average cost to repair 13.5% of damage to the transmission is $1,690. This leaves $4,160 to be distributed among the other three components of the truck.

Since the chassis of the truck is the second least expensive part of the truck at an average of $25,000, I will use it as a backup trigger for the service indicators. To reach 15% of damage after 55,000mi, it must accumulate damage at a rate of 1.69e-6 damage/km. The average cost to repair the chassis will then be $3,750. This leaves $410 for the last two components.
As the chassis should last until a rebuild is required, it will accumulate wear at a very low rate of 5.82e-8 wear/km. It will reach 15% of wear at ~1,600,000mi.

Cabin wear follows the same logic as chassis wear.
As far as damage goes, I wanted to keep the cost down, since there's usually not much to repair except for the occasional rock chip in the windshield or a broken headlight.
With the average cabin costing $30,000 in the game, the $50 repair cost results in 0.17% damage after 50,000mi, or 2.12e-8 damage/km. This leaves $350 in service costs for the engine.

With the average price of a truck engine in the game being $50,000, the service cost of $350 after 50,000mi is equal to 0.7%, or 8.7e-8 damage/km. A typical truck engine service includes oil and filter changes and other minor maintenance.
A typical semi-truck engine, when properly maintained, will last an average of 800,000mi before experiencing more serious problems. [13] The engine wear is therefore 1.17e-7 wear/km.

As calculated above, the service cost for the trailer should be $2,925 per 50,000mi driven. Trailers consist of only two parts for which this service cost applies: body and chassis. Since there are many different types of trailers in the game, average prices for these parts vary widely. Chassis are about the same price on average across all trailers ($26,000), so I put most of the service costs into them to make them more consistent. After 50,000mi, the trailer chassis will have 11% damage, which will cost an average of $2,860 to repair. The trailer body will accumulate 1% of damage within the same mileage and then cost anywhere from $30 to $600 to repair, depending on the type of trailer.
For the wear of both the chassis and body of the trailer, I used the same values as for the truck's cabin and chassis. This way, the trailer will have the same overall life as the truck.

Permanent wear increases by odoemeter_wear_unfixable with every kilometer. As a result, the truck will reach 100% of permanent wear after 1,880,000mi or 3,030,000km. Typical commercial truck life is between 750,000-1,000,000mi. [13] After this distance, the truck usually requires pricey repairs and replacements, which means it's no longer financially viable for most companies and the truck is consequently sold. However, the cabins and chassis of these trucks can still go much further and are thus not in a condition that would require a full restoration.

With the SCS default value, a truck would reach 100% of permanent wear after 3,100,000mi (5,000,000km). But according to a Reddit thread I found, this seems to be quite out of the ordinary and the average mileage seems to be more like 1.8 million miles. [14]
As previously noted, the truck's engine will need to be replaced after ~800,000mi. To determine the point at which the truck should need to be restored, I assumed that a truck should be good to last through two engines in its lifetime, before a restoration is necessary. (This assumes that the truck has always been properly maintained). Since the truck starts to have problems after 15% wear on the engine, I calculated that the permanent wear should be ~85% after 1,600,000mi, which equals 3.3e-7 wear/km. At this point the second engine would have 15% wear and the player would have to choose between continuing to drive with the bad engine or replacing it again, which would not last nearly as long (~300,000mi), or restoring the truck. Or just buy a new truck altogether. Frankly, most players will probably never get to this point, as it would require quite a lot of hours of driving.
The permanent wear value also applies to trailers, as there is no individual variable for trailers.

No changes were made to the variables that control damage from accidents.

Added three previously undefined variables to the police_data.sii file that control the thresholds for when the DOT issues warnings and fines for using a damaged vehicle:
The essence of these changes is that warnings will now appear when any part has 15% or more of total damage. In reality, this could be issued, for example, by the controlling officer who saw that the check engine light was on in your truck.
Fines will be issued if any core part has 50% or more of total damage. (I suspect most players will have repaired any damage before this point, as the damage effects will already have become annoying).
Fines for worn tires will be issued when they exceed 60% of total damage, so get those tires fixed before then.
In addition, the fine for using a damaged vehicle has been reduced to $300.

Since I find the implementation of an insurance in its current state not very realistic, I decided to change it a bit. For one thing, I removed the deductible altogether because it made small repairs quite costly and would always apply. In reality, you probably wouldn't handle every repair through your insurance, but since there's no option in the game for the player to make a choice, I assumed that wouldn't be the norm.
Additionally, I increased the player's share of the repair bill to 90% by default. If the player drives accident free, this percentage can decrease to 70% in about 2 months (in-game time). If the percentage in your savegame is currently lower than this, then it will increase to 70% the next day. The reason I kept the percentage split is so the player can get back some of the insurance money I added to the service cost and has to pay less in case of a major accident. Accidents of any kind will have a greater impact on the percentage than in the original SCS implementation. If you add this mod to an existing savegame, the new insurance values will be applied once the current day in-game has passed.

Here are the changed variables in the bank_data.sii file for anyone interested:

[1] https://forum.scssoft.com/viewtopic.php?p=14943#p14943
[2] https://www.fleetowner.com/operations/article/21676805/tires-closing-the-gap
[3] https://www.tiredeets.com/commercial-truck-tires/
[4] https://business.michelinman.com/tires/michelin-x-line-energy-z
[5] https://ratings.freightwaves.com/cost-to-retread-tires/
[6] https://bettstruckparts.com/truck-repair/alignment/
[7] https://www.mastermobilealignment.com/cost-semi-heavy-duty-truck
[8] https://www.fleetequipmentmag.com/truck-tire-retreading-overview/
[9] https://autoily.com/how-long-do-trailer-tires-last/
[10] https://www.thetruckersreport.com/infographics/cost-of-trucking/
[11] https://blog.freefreightsearch.com/how-many-miles-do-truckers-drive-a-year/
[12] https://www.volvotrucks.us/media/vtna/files/shared/parts-and-services/maintenance-intervals/2021/saag-2017-2020-poster-062221-eng.pdf
[13] https://cararac.com/blog/how-many-miles-can-semi-truck-last.html
[14] https://www.reddit.com/r/Truckers/comments/uk1637/whats_the_highest_mileage_youve_seen_on_a_truck/