Fixing Tire Size by modding Differential Ratios - ETS2

[al_batros]

Fixing Tire Size by modding Differential Ratios - ETS2

For all stock In-Game trucks & transmissions.
(includes the new Scania G33CM 2.59 Gearboxes and fixes the downshift issue at 80km/h!)

Current (1.57) In-Game tire-size has a rolling circumference of 3.1793 meters, which is 2.79% off the
standard being 3.093 meters.

This mod fixes the "tire size" to match ETRTO (European Tire and Rim Technical Organization)
default tiresize for 315/70 R 22.5 by modding the differential ratio of the transmissions.

Each differential ratio of the stock in-game transmissions is increased by 2.79%.

This leads to a few second order effects like engine rpms being 2.79% higher, now better matching
the engine torque curve and increasing the engine power. - Giving us the numbers we should have seen all along!
Drawback due to higher rpms will be a slight increase in fuel consumption.

General data for tire "rolling circumference" in meters:
3.1793: ETS2 faulty 315/70 R 22.5
3.093 : 315/70 R 22.5
3.282 : 315/80 R 22.5
3.184 : 295/80 R 22.5 <- very close to stock SCS tire size!
2.879 : 315/60 R 22.5

As can be seen from the numbers above, ETS2 stock tiresize is only 0.15% off from 295/80 R 22.5.

The ETRTO Numbers can be found the Goodyear European Truck Tires Databook.
Also individual numbers for specific truck tires can be found in the vendor databooks.

For Reference: (all 315/70 R 22.5)
3.093: Goodyear FuelMax D Gen 2
3.095: Goodyear KMax D Gen 2
3.094: Michelin X Line Energy D2

Calculation applied:
Multiplier = 3.1793 / "rolling circumference"
Multiplier = 3.1793 / 3.093 = 1.0279

Conversions applied:
2,17 2.231
2,31 2.374
2,53 2.601
2,59 2.662
2,64 2.714
2,69 2.765
2,71 2.786
2,733 2.809
3,08 3.166
3,36 3.454
3,4 3.495
3,7 3.803
3,91 4.019
3,96 4.070
4,833 4.968


Most likely origin of the bug:
The "rolling circumference" of tires is NOT "pi * diameter", but ""some factor" * diameter".
And SCS applied pi instead of "some factor" (which in this case is 3.05).
Probably "in game" since inception - do I get a bug bounty for finding this?

Whenever SCS comes around to fix this, this mod will be obsolete.

Sharemods:
ETS2_tire_fix-1.0.zip - 123 KB

ATS Version: viewtopic.php?t=347712

Update: Clarification about numbers - others would call it "anchor effect"

[abasstreppas]

So what I understand from this is that a Continental 315/70R22.5 could have a different diameter (which affects the rolling circumference) compared to a GoodYear tyre of the "same" size?

I've always used the https://tiresize.com/calculator/ when calculating the sizes of my tyres and a simple guess is that SCS have done the same.

Guess I have to fix all my tyres SCSoon

A note from Continental Technical Data Book is that on top of the column that displays the rolling circumference, says the value is +-2% If I add 2% to your figure it end up at 3.155 I know it could go the other way too... I've tried to find if that "some factor" is calculated on a non spinning tyre or a tyre spinning at 80 km/h

[al_batros]

So what I understand from this is that a Continental 315/70R22.5 could have a different diameter (which affects the rolling circumference) compared to a GoodYear tyre of the "same" size?

Yes, but "minor". Even within the various tires of a single manufacturer (as I quoted the different values for the GY FuelMax and KMax.) But they all seem to try to adhere to the ETRTO specs.

I've always used the https://tiresize.com/calculator/ when calculating the sizes of my tyres and a simple guess is that SCS have done the same.

Yes.

A note from Continental Technical Data Book is that on top of the column that displays the rolling circumference, says the value is +-2% If I add 2% to your figure it end up at 3.155 I know it could go the other way too... I've tried to find if that "some factor" is calculated on a non spinning tyre or a tyre spinning at 80 km/h

I just crosschecked the Conti databook. On the 315/70 they only quote one number (3093) across their whole 315/70 product range. The +/-2% could therefore also mean that individual models may differ by +/-2% from this 3093.
Whereas Goodyear quotes the ETRTO number and values per model.

Regarding the definition of the rolling circumference:
Conti Databook:

Rolling circumference
is the distance covered by each revolution of
the tyre defined in ISO 9112.

ISO says in its abstract: (https://www.iso.org/standard/45652.html)

ISO 9112:2008 specifies two methods for measuring the rolling circumference and the number of revolutions per unit distance (kilometre) of new commercial vehicle tyres, under loaded conditions, for use on trucks and buses.

The first method is a drum method consisting of loading a test tyre mounted on a free axle against a driven road wheel (or drum) of specified diameter. The second method involves a vehicle whose drive axle is equipped with the test tyres driven on a straight, paved road.

OTOH this "some factor" is defined by the ETRTO as well. Here I have quoted the ETRTO relevant section: viewtopic.php?p=2078938#p2078938 (sometimes referring to a post on the previous page: viewtopic.php?p=2078898#p2078898)

More "general":
What triggered me to investigate all of this was that the numbers I got from the 3.1793 didn´t match what truck tests showed.

Even if German, I try to pick the parts and translate: https://kfz-anzeiger.com/wp-content/upl ... 15_neu.pdf
It´s a test of a Scania R450 Streamline (so a bit older...). The technical data section on page 3 of the pdf (left column) tells us the gearbox spec (final drive 1:1), the axle and diff ratio (2.59:1) and the "Gesamtauslegung" of 1118 rpm at 80kmh.
Middle column - top tells us about the tires and they say they are running Michelin 315/80 front and 315/70 rear / drive. The 315/70 drive, we need to know about.

Now pluggning these numbers in,
[engine rpm] = [kmh] * [gear ratio] * [diff ratio] * [revs per km] / 60
with [revs per km] = 1000 / 3.093 = 323.3 (using the ETRTO-Number)
we get: 80 * 1 * 2.59 * 323.3 / 60 = 1116 being only 2 engine rpms off from the 1118 quoted in the test.
Whereas the SCS numbers would get us to 1086 rpms - the 2.8% this is all about...

The 2.8% that made many pieces of the drivetrain-puzzle fall into its place.
Think of torque-curves that have their peak torque start at 1050. They never properly matched the rpms. Now they do.
Or power requirements like 250+ KW for 40to GVW or 270KW+ for 44to GVW. Now it´s like (80kmh, 2100Nm, direct drive, 2.85, 315/70=270KW, with 315/80=255KW. Boom. Done.) There are guys out there who design drivetrains in a certain way, who match engine torque, revs, power, differentials and tires and the puzzle works. There is beauty in numbers. But in Game it never was.

[abasstreppas]

Thanks for your very detailed answer. Never too old to learn. Cheers

[knox_xss]

Most likely origin of the bug:
The "rolling circumference" of tires is NOT "pi * diameter", but ""some factor" * diameter".
And SCS applied pi instead of "some factor" (which in this case is 3.05).
Probably "in game" since inception - do I get a bug bounty for finding this?

You are mistaken. The rolling circumference is calculated by the game based on the rolling radius, which, in turn, is determined by the position of the bb locator in the wheel model (info).

Another issue is that the rolling radius in the game has been increased by approximately 1.3-1.4 cm compared to the nominal value, and accordingly, the size of the wheels has been slightly increased (why - only SCS knows ).

It is also worth remembering that the rolling circumference (and therefore the rolling radius) for the same tire size may vary depending on the brand and tread model. For example, for a 315/70 R 22.5 tire, it's ~3080–3110 mm.

Here's a comparison of some gaming tires and their bb locator (rolling radius) positions...
(left - default, right - approximate nominal rolling radius)

[ external image ] [ external image ]

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[al_batros]

Many thanks for looking into it!
I will look more into your reply "tomorrow".

For now: Basically, the game is wrong by using "rrolling = 1000 ÷ ( RPK × 2 × π )" because the ETRTO spec defines - and the tire manufacturers apply it as "rrolling = 1000 ÷ ( RPK × 2 × 3.05 )" i.e. they are using 3.05 (for certain tire sizes like 315/70 R 22.5) instead of pi.

Quote from ETRTO spec:

4-Screenshot 2025-11-21 150849.jpg

If SCS wishes to redefine what the tire standards organization does, they might want to give them a call and tell them, how it´s being done
Or the tire diameter / radius for the tires needs to be adapted to match the IRL rolling circumference.

When (see my post above) a real world Scania test drive matches the ETRTO spec by +-2 rpm whereas SCS is 2.8% off, most likely the real world data are the ones we need to look at - and try to replicate as good as possible, don´t we?

[knox_xss]

Using a coefficient (3.05) instead of the mathematical π isn't an error, but an empirical adjustment that takes into account the physics of tire deformation under load. ETRTO introduces such constants to simplify calculations for manufacturers of speedometers, ABS/ESP systems, tachographs, and other devices. In the game, there's no tire deformation under load (and it's not necessary), so the calculation is apparently simplified, using π.

If we plug in the SCS radius value (0.506251) here...

Now pluggning these numbers in,
[engine rpm] = [kmh] * [gear ratio] * [diff ratio] * [revs per km] / 60
with [revs per km] = 1000 / 3.093 = 323.3 (using the ETRTO-Number)
we get: 80 * 1 * 2.59 * 323.3 / 60 = 1116 being only 2 engine rpms off from the 1118 quoted in the test.
Whereas the SCS numbers would get us to 1086 rpms - the 2.8% this is all about

... we get ~1086 rpm (as you wrote). And if we plug in the nominal average value (0.492), we get ~1117 rpm.

Therefore, as you said, it will be sufficient to adjust the wheel models to match the IRL rolling circumference.

In general, judging by what you said earlier...

ETRTO diameter 1.014 (from the Goodyear screenshot) * 3.05 = 3.0927 meters = 3.093, as in the screenshot.

And Goodyear and others seem to work that way.
(The Michelin tire is quoted with 3.094 rolling circumference in their databook.)

... and also by the existing radii in the wheel models (see my screenshots), it seems that SCS used ETRTO data (1.014) when modeling the wheels, while in the game itself the calculations are done using π.

[al_batros]

Hello @knox_xss !

(while you added your 2nd post, I was working on this... so a bit of an overlap

Many thanks again for looking into it and posting the Screenshots!

I have crosschecked what the game is reporting via the SDK:

When I use the f315-70_michelin_xlinez.dlc_michelin as front tire, the SDK reports:

Code: Select all

      {
        "simulated": true,
        "steerable": true,
        "powered": false,
        "liftable": false,
        "lifted": false,
        "radius": 0.5065,  <- THIS
        "position": {
          "x": -1.05,
          "y": 0.506,
          "z": -1.81033218
        }
      },

where the "radius" matches the default value for the bb-locator in your (left) screenshot.

When using "stock SCS tires", the game reports:

Code: Select all

     {
        "simulated": true,
        "steerable": false,
        "powered": true,
        "liftable": false,
        "lifted": false,
        "radius": 0.506,  <- THIS
        "position": {
          "x": 0.916888237,
          "y": 0.506,
          "z": 2.0425632
        }

matching your front_1 (left) screenshot.

Using 0.506 as Radius: 0.506 *2 * pi = 3.1793 - which then matches the SDK reporting:

Code: Select all

"tyreCircumference": 3.17929165457879,

And that´s where IMHO we need to see 3.093 or - as you correctly pointed out - something in the range of ~3080–3110 mm depending on tire manufacturer, but 3.093 being a best match.

If the game were using 3.093, the downshift into overdrive on the new Scania G**cm transmissions at 80 kmh into a "rpm_range_high_gear: (850, 1150)" would work, but it only does so under some rarely matching circumstances, whereas with "real 315/70" there are no issues as the engine revs at 870, leaving a margin of 20 rpms.
[engine rpm] = [kmh] * [gear ratio] * [diff ratio] * [revs per km] / 60
= 80 * 0.78 * 2.59 * 1000 / 3.179 / 60 = 847 rpm with "SCS 315/70".
= 80 * 0.78 * 2.59 * 1000 / 3.093 / 60 = 870 rpm with "real 315/70".
Update: when using this mod:
= 80 * 0.78 * 2.662 * 1000 / 3.179 / 60 = 870 rpm with "SCS 315/70 and this mod".

Which IMHO serves as an experimental proof of the 3.1793 "rolling circumference" being used by the game.

As you point out the bb-locator is the one that needs to be changed, the "approximate nominal rolling radius" in your (right side) screenshots would be what we would need to see in game. That would render my mod obsolete
Could this be done? Maybe just for the stock SCS rear/drive tires? (give them a finger and they want the hand: DLC tires, ATS... )
Blender is beyond my paygrade, that´s why I had to use the /def-transmission route

Edit / Update:
- Most likely front and rear/drive tires need to be modded - for the 4x4 / 6x6 / 8x8 crowd
- Whoever wants to crosscheck the numbers, get the Telemetry Server from https://github.com/PauloTNCunha/Telemet ... ree/master (I am using 4.1.2.3) - install and then open
http:// <localIP> :25555/api/ets2/telemetry (or click link in server application) + there is a "format" checkbox on the top.

Edit 2:
- added calculation for Scania @ 80kmh when using this mod.

[knox_xss]

Hello!

Great idea to use telemetry!

I tried setting the BB locator to approximately the nominal rolling radius according to ETRTO, while also adjusting the wheel sizes... and I came to the conclusion that this was incorrect. Because the wheels are geometrically modeled correctly (and I was wrong to blame SCS). For example, mathematically, the overall diameter of "315/70 R 22.5" would be 1012.5 mm. In reality, it fluctuates between 1007 mm and 1028 mm. In the game, it's currently 1013 mm (for f_315-70_michelin.dlc_michelin) and 1012 mm (for front_1)...
[ external image ] [ external image ]

If adjust the wheel model to the "new" BB locator location, the size is lost - i.e., "315/70 R22.5" no longer represents this tire size, which is incorrect.
If only move the BB locator but leave the model unchanged, then in the game the wheels will appear slightly sunken into the asphalt, since BB, in addition to the radius, also controls collision.

Therefore, your solution is currently the only one. One can only hope that someday SCS will add the "circumference" parameter to the tire defs.

[al_batros]

@knox_xss, many thanks for your efforts and for digging into it!

Let´s see, what solution SCS will come up with - and when

Maybe the truck manufacturers across ATS and ETS2 can put their weight behind an expedited schedule? It´s their drivetrains that don´t work as designed (and expected)!

The cool thing of using the diff ratio is: Any truck can be run with the default set of tires and only the diff ratio needs to be adjusted to get the wanted tiresize! (+Mod the "AB70" Fuel/Rain/Noise specs)

These are my current mulitpliers:
ETS2: (all based on ETRTO)
- 315/70 R 22.5: 1.0280
- 315/80 R 22.5: 0.9688
rarely used:
- 295/80 R 22.5: 0.9985 -> Stock SCS tires are 315/70 on the outside, but "behave" like 295/80!
- 315/60 R 22.5: 1.1045

ATS: (I have opted for GoodYear FuelMax LHD2)
- 295/75 R 22.5: 1.0180 (514 rev per mile - same as Michelin!)
- 11 R 22.5: 0.9804 (495 rev per mile)
- 445/50 R 22.5: 1.0339 (522 rev per mile - FuelMax SSD Duraseal - Drive)

Happy trucking!