S80 Tyre size quandary
 

My wife's S80 (2008 build 3.2 litre AWD) needs 2 new tyres
The tyres fitted by previous owner are
Front 235/40 R18, near new
Rear 245/40 R18 , worn out

The sticker on the door jamb shows 245/40 as the recommended size for 18 inch wheels. Not sure if the 235/40 is an issue but they seem to behave OK on the road

The problem is that if I buy the correct tyre size of 245/40 for replacements of the worn set then I will continue with mismatched tyre sizing. There is a small difference in diameter between 235/40 and 245/40 - about 8mm

I understand that the later generation Haldex as fitted to the 2008 build S80 are not sensitive to tyre size mismatches, but using the wrong size will increase speedo error I assume.
Does the speedo get its reading from the wheel rotations, and if so, is it front or rear?

I've mulled over buying another set of 235/40 so at least then all 4 tyres would be the same size

Also, if I decide to run with mismatched sizes, should I stay with the narrower 235/40 tyres on the front and wider 245/40 on the rear?

Any advice / comments appreciated.

Running 235/40 if not specced for the car, is a modification and affects the insurance from a legal standpoint if they arent notified and have approved it.
As such, I would stay with 245/40 and look to replace the 235 when you can, or, ask the insurance company.
The 235 will be suitable in terms of grip etc, but will make speedo overread.

Bear in mind also the load rating must be suitable for the vehicle at the listed tyre pressures for the vehicle. In principle, the lowest rating listed for the car will be the benchmark as that is suited to the car, thus if the 235 have a lower load index than the 245, but it exceeds a lower load index fitted by manufacturer, then it should be considered appropriate.

My 2007 S80 has 235/40/R18's all around. The previous owner put non-standard alloys on it at some point - I've owned the car for two months. I like the look of them if I'm being honest because they aren't too flash, but they look subtly different at the same time.

However, I knew the rear tyres weren't the best in terms of remaining tread when I bought the car & I knew I had to change them at some point. The rear tyres were a different size to the front - I think they were 235/45/R18 on the rear & 235/40/R18 on the front. I changed the rear ones so the tyres all around are 235/40/R18, as buying a new set of standard alloys & wheels isn't cheap.

Surely 235/40 and 245/40 should be the same diameter?
235/245 refers to width; the 40 profile (in this case) remains the same - therefore not affecting speedo reading.

The profil figures or height of the side wall are a % of the width. IE 40% of 235cm = 94mm and 40% of 245 = 98mm

Corrected

Of course - aspect ratio... doh!! :rolleyes:

Wrong tyres mean not insured as the vehicle is not at manufacture specification that it was designed for on them wheels. Find the correct tyre size and fit them immediately. Any problems phone volvo for advice. When I got my car from a non volvo dealer the rear tyres were 235 instead of 245, pointed it out and was told the car was not road worthy till the correct tyres were fitted, the non volvo dealer had to foot the bill!

I often see the "insurance will be invalid" statement but never seen a case where it has been refused. As far as I know the MOT and vehicle construction regulations only specify the condition of the tyres and that they will be suitable for the load each axle carries.
I have seen many accidents, some fatal, and the police have never been bothered about the tyres unless they are bald or underinflated.

Its the insurance investigator who may make a deal of it, they do make note of things like that.
The load index is the key legal requirement as well as the tread depth, but, where you check 'No' under 'Is the vehicle modified' is a legal statement, if you know that the tyre size differs from those fitted to the vehicle by the manufacturer yet tick this button, that constitutes insurance fraud. The ambiguity comes from either party proving or disproving the intent. However you are required as part of your insurance policy to ensure your vehicle is roadworthy and correctly maintained. Part of this is checking your tyres, of which part of this is ensuring they are in accordance with the vehicle manufacturers parameters, which would involve them being the correct size, pressure, and load index. You may deviate from these if the insurance company approves, but this is classed as modification.

So, whilst some claims may not be affected if tyre size isnt taken into consideration, the distinct and present legal possibility of refusal of policy due to to technicalities does indeed exist, and insurers will generally use any technicality possible to avoid payout.