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Discussion Starter · #1 ·
I still have a very long time to go to replace tyres, but wondered if you would be able to replace the current 225/40R18 with 235/45R18 on the standard Midar or R-Design Volvo rims.

A little bit bigger sidewall and fatter tyre for a smoother ride. The car's arches seem to be able to handle it easily, but would the rim be able to?
 

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I still have a very long time to go to replace tyres, but wondered if you would be able to replace the current 225/40R18 with 235/45R18 on the standard Midar or R-Design Volvo rims.

A little bit bigger sidewall and fatter tyre for a smoother ride. The car's arches seem to be able to handle it easily, but would the rim be able to?
Hi VoodooBob,

Though the tyres may physically fit, the issue is the rolling circumference of the tyre. A change in aspect ratio (the 40 and 45 bit) is a percentage of the width and a wider tyre with a taller aspect ratio will work out as a larger tyre
all round not just its width. This will put the speedo out and then affect things like abs and traction control. There's a little tolerance built-in but I don't know how much Volvo allow. Looking at the sizes you've specified the new tyre works out 4.9% bigger in circumference. This may be too much for the electronics to compensate for. I'd ask Volvo for advice here but I reckon it's too big a change with out re-calibration.
The rule of thumb is if the tyre goes wider, then the aspect ratio should come down and vice versa if the tyre goes smaller.

So ultimately the only way to achieve a taller more comfy tyre is to fit a smaller diameter one or a narrower one but I don't think you'd find one narrow enough but that's the theory
.
Here's a site that provides a calculator.

Hope this helps.
 

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............... This will put the speedo out and then affect things like abs and traction control. There's a little tolerance built-in but I don't know how much Volvo allow. Looking at the sizes you've specified the new tyre works out 4.9% bigger in circumference. This may be too much for the electronics to compensate for. I'd ask Volvo for advice here but I reckon it's too big a change with out re-calibration.
The factor which affects the speedometer accuracy is the dynamic rolling radius. The change contemplated would increase that figure by 5.2%, and hence would make the speedometer read 5.2% slow ~ say 5% in round figures.
However, the tyre suspension comfort value would increase from 2.9 to 5.4 which I reckon would be beneficial. http://www.club80-90syncro.co.uk/Syncro_website/TechnicalPages/TRC calculator.htm

Of course, I could be wrong. :-/
 

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Besides the change in overall tyre diameter and its impact on the speedo (and odometer), also consider any legal ramifications. Certainly here in Australia the increase in tyre OD from 637mm to 669mm would be considered to be a "modification" and as such you may be in trouble with your car insurance. In most Australian states the tyre OD cannot vary by more than +/- 15mm (+15mm / -26mm in some states) from the OEM tyre OD.

The OP may need to check their own local laws.

Tony
 

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I still have a very long time to go to replace tyres, but wondered if you would be able to replace the current 225/40R18 with 235/45R18 on the standard Midar or R-Design Volvo rims.

A little bit bigger sidewall and fatter tyre for a smoother ride. The car's arches seem to be able to handle it easily, but would the rim be able to?
You need to know the width of the rim of the wheel. This webpage gives the minimum, ideal, and maximum tyre width for a range of rim widths. http://www.brick-yard.co.uk/forum/tyre-size-for-rim-width_topic60146.html

Here's another webpage on the same subject. http://www.yokohamatire.com/tires_101/customizing/rim_wheel_width/
 

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You need to know the width of the rim of the wheel. This webpage gives the minimum, ideal, and maximum tyre width for a range of rim widths. http://www.brick-yard.co.uk/forum/tyre-size-for-rim-width_topic60146.html
I Googled for the width of an 18" Midir wheel, and the following webpage suggests that it could be part number 30760138 (or 30760052 or 31373107 depending on the finish) which has a width of 7.5"
http://accessories.volvocars.com/Ac...14/T4/Automatic/R.H.D/ShowDocument/VCC-463317
If that does actually apply to yours then a 235mm wide tyre might be acceptable. However, the technical data on that webpage gives a recommended tyre size of 225/40 R18 92W XL.
To be on the safe side I think you should check with Volvo or your Volvo dealer. Your Volvo dealer should at least be able to give you the part number of the wheel that's fitted to your car, which would make any discussion with Volvo easier. http://www.volvocars.com/za/top/about/corporate/Pages/contact-us.aspx
 

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Discussion Starter · #8 ·
I see the S60 that has 235/40R18 is 8x18, so to be safe it looks like it is better to to go for 8x18 rim on 235 wide profile, instead of 7.5x18

Must get myself a nice set of 19 inch R-Design Ixion rims, now that would looks awesome, but it will slow down my poor little T3 a bit
 

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Discussion Starter · #10 ·
From the list I can see 235/R19 will fit, the only reason I think they are not approved, is because the engine power is not enough, as the extra weight of the bigger wheel will make the car slower. T3 and T4 is the same motor, same body etc, just power is different, one make 110kw and the other 132kw. so if you polestar the T3 up to T4, you fall into the "approved" group again.

Same principle when you have a little 1.2 liter Kia or something that comes standard with 13 inch rims, and now you put on 17 inch rims, that poor little motor is now going to have to work harder to move those tyres.
 
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