Hi,

Earlier this year I bought a 2026 Octavia IV RS Combi, and it came with 225/45 r18 summer tires.
It's this time of year that I have to start thinking about getting a winter setup.
I've noticed that even current gen Golf's use 225/40 R18 tire size, and quite a lot other cars do as well.
I've also noticed that 225/40 R18 size has much more new premium tire models available (like Michelin CrossClimate3 Sport, or Goodyear Vector All Seasons 4),
while they're not available in 225/45 R18 size. And interesting tires that are available in both sizes, like Pirelli SF3 are exactly 50% more expensive in the 45 profile compared to 40 profile.

Needless to say, I see a lot of benefits of going with th 225/40 R18 for my winter tires setup, but that will give me over 3% smaller wheel diameter.
Therefore does anyone know if that will work?
Won't abs/esp go crazy?
Is it possible to modify the wheel diameter in the car computer using a new OBD Eleven interface perhaps?

If the tire size is not specified in the car's official papers (e.g. Certificate of Conformity), just don't use that size - there are plenty of reasons for that.
BTW - if you're looking for a winter setup, why are you searching for All Season tires?

All Season 3PMSF (Three Peak Mountain Snowflake) tyres might be all that is needed if you do not require FULL SNOW Tyres. Plenty All-Seasons suitable for use in Snow or pisted or non pisted roads and even if Snow Chains are needed. @Mariosti The ABS / TPMS / TC will not give a monkeys if there are different widths or circumference of tyres fitted. Reset the TPMS. But you are intending changing the Diameter / Circumference. Maybe just fit narrower treads / sidewall to sidewall if cost is an issue. Set your Winter Tyre speed warning on the car if Winter / Snow Tyres are fitted.

Edited 13 hours ago13 hr by Evolution13

If you're interested in changing the tyre to a smaller size, keep in mind that the ADAS systems might use the wheel circumference as an input to calculate the vehicle speed. While a 3.5% difference doesn't seem that much, it might be too much of a difference for the systems to compensate it. Then you'll end up with weird behavior when driving, due to the wrong vehicle speed calculations

A way to avoid this would be to code the new wheel circumference to the vehicle, but this implies a dealer visit, which might refuse to do it (since it's not a size homologated for your vehicle)

Below you can see a comparison between the homologated sizes for the RS

6 hours ago, SkOmk4 said:

If the tire size is not specified in the car's official papers (e.g. Certificate of Conformity), just don't use that size - there are plenty of reasons for that.
BTW - if you're looking for a winter setup, why are you searching for All Season tires?

In very recent tire review tests those All Seasons tend to get within 5% off the best premium winter tires (like Michelin Alpin 5) in various snow traction tests, or even on par in some specific aspects (like ice traction). While on the wet they're vastly superior (10-20% better in aquaplaning, breaking, handling) than winter tires.
So while I'll certainly use them in the winter in the mountains in proper snow for a few weeks each year, most of the time it would be the wet conditions to worry about, and unfortunately pure winter tires aren't really that good in the wet.

All seasons will be also best for me, because then I can use the summer set to be track day specific, and I already did an hour on the Nürburgring this summer, and it does have the performance to eat up tires, so this way I wouldn't need 3 sets of wheels, or joggling tires.

6 hours ago, Evolution13 said:

All Season 3PMSF (Three Peak Mountain Snowflake) tyres might be all that is needed if you do not require FULL SNOW Tyres. Plenty All-Seasons suitable for use in Snow or pisted or non pisted roads and even if Snow Chains are needed. @Mariosti The ABS / TPMS / TC will not give a monkeys if there are different widths or circumference of tyres fitted. Reset the TPMS. But you are intending changing the Diameter / Circumference. Maybe just fit narrower treads / sidewall to sidewall if cost is an issue. Set your Winter Tyre speed warning on the car if Winter / Snow Tyres are fitted.

Well, the actual point behind going 225/40 r18 would be not about some snow traction shenanigans, but rather having the same tire and wheel size for track day set, and winter set, so that rims clear the breaks, the speedo properly set once, and at the same time I can get the lightest wheels possible without breaking the bank. That's the main reason why I was thinking about it. 19 inch rims are noticeably heavier, and again, 225/40 r19 has even worse tire selection than 225/45 r18, and are also significantly more expensive for the same tire model - so not much sense to pay significantly more for a heavier rim and a heavier tire with even more inertia in a performance car.

I was after the 225/40 r18 Goodyear Vector All Season 4, because they got measured at just 8.1kg each, while other tires in that size are near 9 or even 10 kg each, and 225/45 r18 ones are obviously a bit heavier still.

5 hours ago, evilsheep said:

If you're interested in changing the tyre to a smaller size, keep in mind that the ADAS systems might use the wheel circumference as an input to calculate the vehicle speed. While a 3.5% difference doesn't seem that much, it might be too much of a difference for the systems to compensate it. Then you'll end up with weird behavior when driving, due to the wrong vehicle speed calculations

A way to avoid this would be to code the new wheel circumference to the vehicle, but this implies a dealer visit, which might refuse to do it (since it's not a size homologated for your vehicle)

Below you can see a comparison between the homologated sizes for the RS

That's a good point. And I double checked that OBD11 still doesn't have SFD2 unlock, and besides the car is under warranty, so fiddling with it could be quite consequential.

I'll just got with the stock size Pirellis SF3 then.

Perhaps I'll get back to this topic in two years time after the warranty runs out, and hopefully OBDEleven gets the sfd2 unlock working.

7 minutes ago, Mariosti said:

Perhaps I'll get back to this topic in two years time after the warranty runs out

Does the warranty cover tyre shredding hours around the 'Ring 🤔🤣

@Lee Public Road and not Racing or Timing. @Mariosti The cars speedometer from the Factory over reads and a slight circumference increase usually has the speed more accurate. Very few would change settings in the ECU for New Tyres with more tread and run at higher pressures or for more worn and lower pressures. various circumferences. PS, Winter Tyres / All Season for road, none track use just getting about, does the UNSPUNG weight really matter. ?? PS. I have had in the past few years Alpin 5 & Alpin 6 and various CrossClimates, First Edition, + and SUV and have CrossClimate 3 on my EV right now. I should have gone with Pirelli Cinturato AS SF3 Self Sealers as i have no spare wheel. Had 2 punctures and replace 1 CrossClimate 3 already in the first 3 months.

Edited 6 hours ago6 hr by Evolution13

4 hours ago, Lee01 said:

Does the warranty cover tyre shredding hours around the 'Ring 🤔🤣

Pretty sure no warranty covers tyres or any consumables on the car like brakes, fluids, oils, filters, shocks, suspension joints etc.
Additional insurance "warranties" may cover, or partially cover such things, but that's quite some extra money upfront each year, especially on a brand new car.

3 hours ago, Evolution13 said:

@Lee Public Road and not Racing or Timing. @Mariosti The cars speedometer from the Factory over reads and a slight circumference increase usually has the speed more accurate. Very few would change settings in the ECU for New Tyres with more tread and run at higher pressures or for more worn and lower pressures. various circumferences. PS, Winter Tyres / All Season for road, none track use just getting about, does the UNSPUNG weight really matter. ?? PS. I have had in the past few years Alpin 5 & Alpin 6 and various CrossClimates, First Edition, + and SUV and have CrossClimate 3 on my EV right now. I should have gone with Pirelli Cinturato AS SF3 Self Sealers as i have no spare wheel. Had 2 punctures and replace 1 CrossClimate 3 already in the first 3 months.

Sorry to hear you got so unlucky with the flats lately.
I'm quite certain that if you bought a spare wheel and put it in the boot then you wouldn't get flats anymore ;)

As to the unsprung weight, I am replacing the 18" skoda comet rims that are 13.2kg each according to Skoda with jrwheels SL01 18x8 , that should weigh around 8.3kg each, so that will be already a huge reduction of almost 20kg of unsprung mass on the car, but if it would be possible to remove additional 4-6kg and also get same premium threads but cheaper - well that would be a win win scenario, and certainly can't hurt also the daily driving.

As a matter of fact i planned to get 5 rims and tires, to get a full size spare wheel. As I understand it should fit in the spare wheel well, and the canton sub should fit inside of the rim, or am I mistaken?

Going back to the main topic, indeed, the circumference difference is too big for daily use, considering all the electronics on the car.

Edited 2 hours ago2 hr by Mariosti

@Mariosti I thought you started out bothering about splashing some cash on tyres that cost a bit more for winter use,. now there is this stuff about different wheels, weight saving. You seem to have all the answers and knowledge so seems funny you asked in the first place.

6 minutes ago, Evolution13 said:

@Mariosti I thought you started out bothering about splashing some cash on tyres that cost a bit more for winter use,. now there is this stuff about different wheels, weight saving. You seem to have all the answers and knowledge so seems funny you asked in the first place.

Well the thing I don't know for sure is how badly the electronics might react to the smaller wheel diameter, and don't really want to waste money on "incompatible" tires.
On pre face-lift models it's apparently not a big deal according to some threads, and the wheel size change in the computer is quite easy and painless (within some limits of course of supported wheel sizes, but there are usually way more sizes programmed than just the two oem sizes for this specific Octavia version, at least according to VW threads). Although looking at octavia spec the smallest oem wheel is the 205/60 r16, and that's 1.12% smaller than the 225/45 r18 - so quite close still, and it may be that the computer/clusters don't have a small enough tire circumference profile available even if it was possible to modify those settings.

I assume that that setting, as it's exactly what the Authorized Service Center changes when changing tire size within oem options, propagates to all the modules that need this information, as even within the OEM tire sizes, they span from 205/60 R16 with 652mm circumference, all the way to 225/40 r19 with 662mm circumference, so over 1.5% difference - still possibly significant for all those advanced electronics, assists, radars etc.

So unless the dials module has some Golf profiles in there, this may not be possible at all, and an experiment will be necessary.

Thankfully that can be done as soon as sfd2 get's unlocked, without the need to physically swap the tires, as the setting change will change the indicated speed, and that's enough to calculate what circumference each profile corresponds to.

Would be nice though if all of that was just openly available in the user manual for the car. Or perhaps some "advanced user" manual - optional.

The Authorised Service Centre or the Tech / Fitter doing a PDI changes nothing in the settings. Maybe Takes it out of 'In transit setting'. Might change Variable Servicing to Fixed Servicing, and might reset the TPMS when they change the pressure from that they come off a transporter at.

1 hour ago, Evolution13 said:

The Authorised Service Centre or the Tech / Fitter doing a PDI changes nothing in the settings. Maybe Takes it out of 'In transit setting'. Might change Variable Servicing to Fixed Servicing, and might reset the TPMS when they change the pressure from that they come off a transporter at.

As "settings" i mean the gauges cluster module using long coding over OBD interface.
Just as the OBDEleven threads for Octavia Mk.4 before FL, and for Golf mk 7 and 7.5 are stating.
Or is the ASC doing it in a different way?

Edited 43 minutes ago43 min by Mariosti