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Tyre sizes and departing from OEM spec

Sue Pertwee-Tyr

Sue Pertwee-Tyr

Accuphase all the way down
I'm having a little trouble finding the truth behind an assertion I read recently, that any variation in diameter of >3% from the OEM spec of your car, is illegal. Can anybody help? Is this correct, or another of those 'every fule kno' things?

My car has OEM spec tyres of 205/40 R17 section, on which the ride is brutal (bloody Audi 'sports' suspension setup on a light car). I changed, years ago, to running 205/45 on the same rim, which improved things significantly and has no obvious downside apart from a slight risk of rubbing at full-lock. This represents a little over 3% increase in rolling diameter.

When I first changed them, I checked the speedo calibration against a known accurate GPS source and it was spot-on at all speeds from 20-70mph, against a 2-3mph over-read previously.

Some sources (eg Uniroyal website) seem to suggest that 1.5-2.5% is all that is permissible due to the risk that bigger deviation may upset calibration of ESP/ABS systems. I can see that unexpected differential between axles might confuse a system, but if both axles change by the same amount, the ESP/ABS system surely won't notice?

No source I've seen has suggested there's anything inherently unlawful about running non-OEM sizes, and as the speedo is accurate I can't see any particular issue the law would have here. My part-worn tyres have lost around 4-5mm of tread anyway, so AFAICT the actual diameter will now be within what would be permissible deviation from the standard size anyway and quite possibly smaller than brand new tyres of OEM spec with the full 8mm tread depth.

I've never had an issue at service/MOT time, but have been left wondering if I should revert - the tyres will need replacing later this year anyway.

Any thoughts?
 
I’ve previously done this to run larger profile all terrain tyres, no issues. What you must do is gain permission from your insurance company to run a non OEM size to prevent any problem in the event of an accident / claim.

You could also buy some 16 inch wheels (or even 15’s if they’ll fit over your brake callipers) and run a higher profile tyre.
 
I ran a slightly taller profile on a set of 19's on my Audi to increase the comfort. I had no issues overall. I spoke to my insurance company about it, and they said as long as the wheel size was a factory spec wheel (which it was being an 'optional extra') the tyre size was irrelevant and not classed as a modification.

I did revert back to 18's though in the end, as the handling was much better with the 19's, I couldn't live with the ride quality as I don't have Magneride. I even replaced the Dampers to Koni Special Actives, which made everything much better, but still not enough.

The 18's with the Koni's is absolute bliss, so so so so much better than the Audi S-Line suspension. If you want comfort and increased handling (none of that S-Line crashy suspension) opt for a set of Koni Special Actives. They are classed as OEM replacements therefore not technically a modification.
 
flutteringwow said:
I ran a slightly taller profile on a set of 19's on my Audi to increase the comfort. I had no issues overall. I spoke to my insurance company about it, and they said as long as the wheel size was a factory spec wheel (which it was being an 'optional extra') the tyre size was irrelevant and not classed as a modification.
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Thanks, that was my understanding. I didn't inform my insurers at the time as my understanding was that this didn't fall within the definition of the sort of modification that required notification. Perhaps I'd have been better advised to do so for peace of mind if nothing else.
 
Tony Lockhart said:
@Sue Pertwee-Tyr

Are the current tyres runflats?
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No, the car's a 2004 Audi A2. Runflats weren't an option, AFAIK. It's more that the tyre width is 'only'* 205mm which makes the sidewall very shallow on a 40 profile tyre. The car is also very light, so there's little by way of mass to help compliance and the road spring rates are arguably a bit too strong. Some ruts and potholes left me convinced the rim had been impacting, or at least the tread smashing up to the bead (and a distorted rim that occurred early in my ownership did nothing to dispel that suspicion). I used to find myself constantly scanning the road surface and, as well as potholes, avoiding grids and manholes unless they were fully flush. Aside from being tiring, it took attention away from other road hazards. On balance, I think safety and security were better served by the change I made.

*times change, a Lancia HPE Volumex I once had ran on 185 section, which looked chunky for the day, but decidedly skinny now. My old Opel Monza ran 205 section, on a car which weighed about 2/3 as much again as my little Audi.
 
My last three cars have had 225/40x18, 225/45x17, and 235/35x19 respectively. I found the 40s to be on the ragged edge of usability on surface streets that may include the occasional pothole or speed bump. When I went from them to the 45s, I thought the 45s were too "floppy" for a car that gets around corners via high grip rather than handling. The present car with 35s is on the wrong side of the usability limit.
 
Sue Pertwee-Tyr said:
Thanks, that was my understanding. I didn't inform my insurers at the time as my understanding was that this didn't fall within the definition of the sort of modification that required notification. Perhaps I'd have been better advised to do so for peace of mind if nothing else.
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If you deviate from the wheel / tyre size combinations approved for the vehicle on the door plate, you need to speak to your insurer.
 
Yank said:
My last three cars have had 225/40x18, 225/45x17, and 235/35x19 respectively. I found the 40s to be on the ragged edge of usability on surface streets that may include the occasional pothole or speed bump. When I went from them to the 45s, I thought the 45s were too "floppy" for a car that gets around corners via high grip rather than handling. The present car with 35s is on the wrong side of the usability limit.
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My C-Class estate has 225/45's on the front and 245/40's on the back and that seems to work fine for potholes etc. (maybe crashing into potholes is more noticeable on the front than the back) although I don't think I'd want anything lower profile that that, especially with the firmer suspension of the AMG spec. Overall it's pretty good mix of handling and comfort - for an estate car anyway. My wife's SLK's is 225/40 on the front and 245/35 on the back I think (and is also the AMG spec suspension) and in practice that's also been a decent balance of comfort & handling. While small that's still a pretty heavy car though, which might make a difference.

When I was thinking about winter tyres for my C-class I was looking at the other fitments for the C-class estate as size options (as I was thinking about getting wheels as well) and I wouldn't have thought twice about going with a higher profile tyre if doing that - but I would have stuck to sizes that were used on the car as I'd have been concerned about the implications for insurance etc. if using a non-recommended tyre size if I was in an accident.
 
SteveG said:
When I was thinking about winter tyres for my C-class I was looking at the other fitments for the C-class estate as size options (as I was thinking about getting wheels as well) and I wouldn't have thought twice about going with a higher profile tyre if doing that - but I would have stuck to sizes that were used on the car as I'd have been concerned about the implications for insurance etc. if using a non-recommended tyre size if I was in an accident.
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We have a C Class with the same size tyres as yours. A year or so after buying it we bought a set of winter wheels and tyres from Mercedes. They’re still 17” rims, but I can’t remember the width and profile. Definitely not 245mm wide!
 
I wouldn't worry about it, you might also be able to run reduced tyre pressures to improve the ride.

If the tyres wear faster in the middle definitely drop the pressure a bit
 
Ponty said:
If you deviate from the wheel / tyre size combinations approved for the vehicle on the door plate, you need to speak to your insurer.
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Selection of any wheel option that is not part of the spec for that model / version, should be notified to the insurer even if it is the same size.
 
Given S-Line suspension has a rep for being something to rattle your fillings out with, if you buy a new Audi S-Line, you can select Sport (was SE) suspension as a no-cost option.
 
Tony Lockhart said:
We have a C Class with the same size tyres as yours. A year or so after buying it we bought a set of winter wheels and tyres from Mercedes. They’re still 17” rims, but I can’t remember the width and profile. Definitely not 245mm wide!
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There seem to be loads of fitments for the C-class - so many that there isn't anywhere on the car or even in the manual that gives tyre sizes. From a quick google it looks like the options for the C200 were:
  • 195/60 x 16 all round
  • 205/55 x 16 all round
  • 225/50 x 16 all round
  • 225/45 x 17 all round (this might be what you have Tony)
  • 225/45 x 17 front, 245/40 x 17 rear
  • 225/45 x 18 front, 245/40 x 18 rear (which is what my car has)
  • 225/45 x 18 all round
  • 225/40 x 18 all round
I think some of the more powerful models had 19" and even 20" wheel options as well. Which is quite a range. I'm assuming the 16" are maybe the winter tyre options but I think there should be something for everyone in there!
 
Ponty said:
If you deviate from the wheel / tyre size combinations approved for the vehicle on the door plate, you need to speak to your insurer.
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OK, so I've now spoken to them (Direct Line) and explained that, on the OEM wheels I've changed from 205/40 R17 to 205/45 R17. They have confirmed to me that, as I had previously understood to be the case, this is not a material change and does not affect the policy. They did however thank me for letting them know, and have noted this on the policy anyway.

So, peace of mind on that front, anyway. :)
 
Sue Pertwee-Tyr said:
OK, so I've now spoken to them (Direct Line) and explained that, on the OEM wheels I've changed from 205/40 R17 to 205/45 R17. They have confirmed to me that, as I had previously understood to be the case, this is not a material change and does not affect the policy. They did however thank me for letting them know, and have noted this on the policy anyway.

So, peace of mind on that front, anyway. :)
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Worth having it noted in the interests of ‘no surprises’. I’ve never had an issue obtaining their agreement or been charged any extra.
 
SteveG said:
There seem to be loads of fitments for the C-class - so many that there isn't anywhere on the car or even in the manual that gives tyre sizes. From a quick google it looks like the options for the C200 were:
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On my car, there’s a small choice of sizes on the inside of the fuel filler cap.
 
Sue Pertwee-Tyr said:
OK, so I've now spoken to them (Direct Line) and explained that, on the OEM wheels I've changed from 205/40 R17 to 205/45 R17. They have confirmed to me that, as I had previously understood to be the case, this is not a material change and does not affect the policy. They did however thank me for letting them know, and have noted this on the policy anyway.

So, peace of mind on that front, anyway. :)
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This is the answer. If you are sensible you can select one of the other standard fit sizes with sensible tyre ratios, they are generally cheaper and ride better. My car has 255/35 x19, it rides like a skateboard on a ploughed field and I'd swap it tomorrow for a more forgiving size.
 
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