235/50R18 Tyres

[XTRBeast]

Hi

I am considering putting some 235/50R18 Michelin CrossClimate 2 tyres on my C-HR (the current tyres are the original 225/50R18 size).

Does anyone have any experience or advice on this idea?

Thanks

[FROSTYBALLS]

Use one of the tyre check websites to determine whether the change in size affects the speedometer readings. Insurance will also need to be informed.

[XTRBeast]

11 minutes ago, FROSTYBALLS said:

Use one of the tyre check websites to determine whether the change in size affects the speedometer readings. Insurance will also need to be informed.

The speedometer would read - 1.44% too slow, about 1 mph.  Considering the speedometer currently reads about 3mph too fast (according to my SatNav and the local speed detection warning sign), I didn’t think that would be a problem?  Yup, would let the insurance company know.

[Mojo1010]

Then go for it

[Haliotis]

It is illegal for a speedometer to red lower than true speed. As you are intending to fit tyres that are slightly wider, it would be advisable to ensure that there can be no rubbing, lock-to-lock.   Since the planned tyre width is only about 10mm wider than the OE, is it worth going down that route?

[DAN@ADRIAN FLUX]

Hi.

Hopefully you won't but if you do have any issues with insurance at all for a change of tyres then please feel free to drop me a line.

Regards,

Dan.

[Aygolite]

On 11/1/2023 at 7:58 PM, Haliotis said:

It is illegal for a speedometer to red lower than true speed. As you are intending to fit tyres that are slightly wider, it would be advisable to ensure that there can be no rubbing, lock-to-lock.   Since the planned tyre width is only about 10mm wider than the OE, is it worth going down that route?

The actual inflated width of a tyre always depends on the rim width to which it is fitted.

As you can see in the chart below, an inflated 235/50R18 when fitted to a 7J rim is only 6mm wider than an inflated 225/50R18 when fitted to a 7J rim.

Interestingly, if the 225/50R18 was fitted to a 7.5J rim, the actual inflated width would be 239mm, ie. the same as a 235/50R18 fitted to a 7J rim.

Tyre width is measured from the outside of one sidewall to the outside of the other sidewall. Tyre width is not the width of the tread that is in contact with the road.

Actual inflated width of tyre when fitted to 7J rim

225/50R18 233mm

235/50R18 239mm

[XTRBeast]

18 minutes ago, Aygolite said:

The actual inflated width of a tyre always depends on the rim width to which it is fitted.

As you can see in the chart below, an inflated 235/50R18 when fitted to a 7J rim is only 6mm wider than an inflated 225/50R18 when fitted to a 7J rim.

Interestingly, if the 225/50R18 was fitted to a 7.5J rim, the actual inflated width would be 239mm, ie. the same as a 235/50R18 fitted to a 7J rim.

Tyre width is measured from the outside of one sidewall to the outside of the other sidewall. Tyre width is not the width of the tread that is in contact with the road.

Actual inflated width of tyre when fitted to 7J rim

225/50R18 233mm

235/50R18 239mm

Great information, thanks.  😀👍

[XTRBeast]

On 11/1/2023 at 7:58 PM, Haliotis said:

It is illegal for a speedometer to red lower than true speed. As you are intending to fit tyres that are slightly wider, it would be advisable to ensure that there can be no rubbing, lock-to-lock.   Since the planned tyre width is only about 10mm wider than the OE, is it worth going down that route?

As the speedometer is reading 3mph too fast as standard, reducing that by 1mph to 2mph too fast should not be an issue.  My main reasons for considering going up a tyre size/width are that 235/50R18 tyres are, for some reason, cheaper to buy than 225/50R18s, also the extra few mm on the sidewall width gives the alloys a bit more protection when kerbs attack.  😊👍

[TonyHSD]

Changing tyres sizes while retaining the original wheels sizes is not ideal. Usually this change is correct when you change wheels and tyres accordingly and those been one of the sizes in your manual handbook or door label. Saving £50-100 from a set of tyres and going different size tyres can lead to complications afterwards. Speedo will change slightly.
Car dynamics changes too. In an event of a serious accident and car been investigated by specialists they will see it and may put blame on that exactly.  
Many people does that, but it is not the right thing to do. Any changes bring positives but also negatives. And if you only change tyres once every few years there is absolutely no point of this hassle. That’s all is only my personal opinion. 

[XTRBeast]

2 hours ago, TonyHSD said:

Changing tyres sizes while retaining the original wheels sizes is not ideal. Usually this change is correct when you change wheels and tyres accordingly and those been one of the sizes in your manual handbook or door label. Saving £50-100 from a set of tyres and going different size tyres can lead to complications afterwards. Speedo will change slightly.
Car dynamics changes too. In an event of a serious accident and car been investigated by specialists they will see it and may put blame on that exactly.  
Many people does that, but it is not the right thing to do. Any changes bring positives but also negatives. And if you only change tyres once every few years there is absolutely no point of this hassle. That’s all is only my personal opinion. 

Thank you, that is an interesting point of view.  I am interested in how tyres that are 6mm wider than factory fit will change the dynamics of the car.  Can you elaborate as I am always willing to learn?

[XTRBeast]

18 hours ago, DAN@ADRIAN FLUX said:

Hi.

Hopefully you won't but if you do have any issues with insurance at all for a change of tyres then please feel free to drop me a line.

Regards,

Dan.

Hi Dan,

do you have any experience or additional advice regarding this change in tyre size from factory, from an insurance point of view?

Thanks. 😊

[TonyHSD]

1 hour ago, XTRBeast said:

Thank you, that is an interesting point of view.  I am interested in how tyres that are 6mm wider than factory fit will change the dynamics of the car.  Can you elaborate as I am always willing to learn?

Hi Mark and thank you for your response. 
The new tyre size 235/50 18 is 10 mm wider than 225/50 18 and around 6mm gains in side wall. This side wall height changes the outside diameter of the tyre and you get 5mm extra height, ground clearance, this  can make the car more unstable in turns. Also the acceleration, stopping will be slightly affected along with fuel efficiency. The transmission ratios, the abs, the stability control, the suspension set up ( shock absorbers and springs, the wheel alignment- camber, caster, ) are all designed with particular wheels and tyres sizes and any deviation from the original set up can and will affect the car behaviour. There are many sites online that offer better and more thorough explanation with graphics how small tyre size change affects all that. If you search Google you will find helpful information.

Regards 
 

[XTRBeast]

6 hours ago, TonyHSD said:

Hi Mark and thank you for your response. 
The new tyre size 235/50 18 is 10 mm wider than 225/50 18 and around 6mm gains in side wall. This side wall height changes the outside diameter of the tyre and you get 5mm extra height, ground clearance, this  can make the car more unstable in turns. Also the acceleration, stopping will be slightly affected along with fuel efficiency. The transmission ratios, the abs, the stability control, the suspension set up ( shock absorbers and springs, the wheel alignment- camber, caster, ) are all designed with particular wheels and tyres sizes and any deviation from the original set up can and will affect the car behaviour. There are many sites online that offer better and more thorough explanation with graphics how small tyre size change affects all that. If you search Google you will find helpful information.

Regards 
 

Wow, very interesting information.  I must have been really lucky that it improved the stability and handling of my previous car when I went from 215/55R18 to 235/50R18.  This is why I wondered if anyone had any actual experience of running different tyres as it seems that you never can tell what the results might be.  Thanks again for the information.  😊👍