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 6 hours ago6 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 19 minutes ago19 min by Evolution13