Using a non recommended tire size

[Easterlily]

Hello fellas.
I am planning to purchase a new set of wheels - considering going for 18 inches to replace the standard 17. 225/40/18 is the recommended tire size for the car with a tire of this size (or 255/35/18 but thats overkill), but I was thinking about, would it be a good idea to get 225/45/18 for a little bit of extra rubber. The advantages in my mind is a slightly more gentle ride, slightly less noise and a bit more filler in the wheel arch as there is quite a bit of an air gap in there. Or am I a complete idiot and am ruining the cars looks and/or steering/suspension function by doing this?

[Mincey]

As someone who has gone from a 17" shod IS250 to an 18" shod IS300h, I'd stick with your 17" wheels. 

[royoftherovers]

14 minutes ago, Easterlily said:

Hello fellas.
I am planning to purchase a new set of wheels - considering going for 18 inches to replace the standard 17. 225/40/18 is the recommended tire size for the car with a tire of this size (or 255/35/18 but thats overkill), but I was thinking about, would it be a good idea to get 225/45/18 for a little bit of extra rubber. The advantages in my mind is a slightly more gentle ride, slightly less noise and a bit more filler in the wheel arch as there is quite a bit of an air gap in there. Or am I a complete idiot and am ruining the cars looks and/or steering/suspension function by doing this?

Just follow what is in the Handbook Jonatan.

[PCM]

14 minutes ago, royoftherovers said:

Just follow what is in the Handbook Jonatan.

+1

Insurance issues?

Mechanical issues?

Your call of course, but I wouldn't... 

Maybe change the tyres, not the wheels?

[Easterlily]

4 hours ago, Mincey said:

As someone who has gone from a 17" shod IS250 to an 18" shod IS300h, I'd stick with your 17" wheels. 

Howcome? 

[Easterlily]

4 hours ago, PCM said:

+1

Insurance issues?

Mechanical issues?

Your call of course, but I wouldn't... 

Maybe change the tyres, not the wheels

According to Swedish law you are allowed to have different size tyres than the recommendations if it doesn't change the wheel circulation distance more than 5%. (and some other rules about how it fits in the well etc.). I am certain a change of only the profile with one step would not give me any insurance or legal issues. 

Mechanical? That depends on if the slightly larger wheels interfere with steering or suspension which is kind of what I want to learn from you guys. 

Also if the slightly larger wheels would look stupid. Maybe it's just me who thinks the wheels have a lot of space around them compared to similar cars. 

[Barry14UK]

In the UK (at least), you should tell your insurers of any change as I did when a similar situation arose with a previous car I insured.  There are sites where you can feed in the tyre size of your standard approved tyres and the proposed ones and it gives the difference between them in distance per revolution as a percentage.  This should be within certain parameters (which I can't recall immediately, as among other things speedo readings can be unacceptably off.)  

[NemesisUK]

4 hours ago, Easterlily said:

Also if the slightly larger wheels would look stupid. Maybe it's just me who thinks the wheels have a lot of space around them compared to similar cars. 

If you increase the wheel diameter and reduce the tyre profile, as you would need to to keep the rolling circumference  and speedometer accuracy within legal requirements, the actual wheel+tyre size will be very similar to the original and so will not fill the arch any better?

Just about the only way of reducing that wheel to arch space is to lower the car on it's suspension (another can of worms...) and to fill the arch more is to fit wheel spacers to bring the wheel/tyre edge closer to the arch outer line (yup, another can opened...)

[Mincey]

6 hours ago, Easterlily said:

Howcome? 

The ride in my IS300h is significantly worse than the ride in my IS250. The road past my local hospital has been dug up more times than I can remember. My IS250 would waft over the road scars. My IS300h rattles and bangs over all of them. I'd swap the 18's for 17's in a jiffy. Ok, it probably corners better than my IS250 did, but I'm 52 and I'll never be Ayrton Senna.

[Pink]

13 hours ago, NemesisUK said:

and to fill the arch more is to fit wheel spacers to bring the wheel/tyre edge closer to the arch outer line (yup, another can opened...)

Was also going to raise spacers as a suggestion, but conscious of the worms in that can !

It’s strange in a way, as I ran spacers for years on my 5 series, as it seemed did almost every other member of the forum at the time. You could guarantee posts on the best sizes and set-ups on an almost daily basis. Notwithstanding the potential stresses on suspension linkages and geometry it seemed to be an accepted given, and must admit I personally never had any issues. 

Yet the IS has a huge gaping void in its arches (especially the 17s on my Exec) so I’m surprised the spacers query doesn’t appear more often. I even have a spare set of Eibach 20mms in the garage but have never put them on the IS. Perhaps the additional Battery weight over the rear accentuates the stresses in a hybrid setup. 

[royoftherovers]

22 minutes ago, Pink said:

Was also going to raise spacers as a suggestion, but conscious of the worms in that can !

It’s strange in a way, as I ran spacers for years on my 5 series, as it seemed did almost every other member of the forum at the time. You could guarantee posts on the best sizes and set-ups on an almost daily basis. Notwithstanding the potential stresses on suspension linkages and geometry it seemed to be an accepted given, and must admit I personally never had any issues. 

Yet the IS has a huge gaping void in its arches (especially the 17s on my Exec) so I’m surprised the spacers query doesn’t appear more often. I even have a spare set of Eibach 20mms in the garage but have never put them on the IS. Perhaps the additional battery weight over the rear accentuates the stresses in a hybrid setup. 

The people who by and large,had Lexus were in a different age group to those that drove BMW  and had no need to acquire and maintain street cred Gareth ?

[Pink]

1 hour ago, royoftherovers said:

The people who by and large,had Lexus were in a different age group to those that drove BMW  and had no need to acquire and maintain street cred Gareth ?

😀 Maybe John, maybe. Although I think it might take more than a couple of wheel spacers for me to acquire any ‘street cred’ personally !!