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235/45/17 on the rear

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Just wondering what you guys think about 235s on the rear.

Just to add some context:

  • the rims are the same for R17
  • IS250AWD runs 225s all round
  • my 225 fronts worn ~1.5mm more than 245 rears 

I am not sure I would do it, but the goal would be better match the rear tire so that it would last about the same as front. In my opinion Lexus overestimated the wear/load on rear and 245s are simply overkill. It was same last time - fronts worn to bits, but rears still hanging over 3mm.

Second obvious benefit 235s costs about the same as 225s, whereas 245s are good £40 more.

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Just wondering what you guys think about 235s on the rear.
Just to add some context:
  • the rims are the same for R17
  • IS250AWD runs 225s all round
  • my 225 fronts worn ~1.5mm more than 245 rears 
I am not sure I would do it, but the goal would be better match the rear tire so that it would last about the same as front. In my opinion Lexus overestimated the wear/load on rear and 245s are simply overkill. It was same last time - fronts worn to bits, but rears still hanging over 3mm.
Second obvious benefit 235s costs about the same as 225s, whereas 245s are good £40 more.
Should be fine.
There's potentially less grip and maybe a rougher ride, but I did something similar by moving from 255/40R18 to 245/40R18 and haven't noticed much difference.
Actually, the one thing I have noticed, is that understeer is reduced. That annoying issue that causes the front tyres (only certain brands i guess) to scrub and skip when they're cold is almost non existent.
I saved around £120 by going narrower!

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Interesting. Always struck me as an odd decision by Lexus.

When I was close to buying an IS250, I tried pricing tyres and found it frustrating how difficult it was to get the tyres I wanted for both front and back. It doesn't seem to me an unreasonable step, but I wonder how do insurers feel about it?

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I reckon ... indifferent, but don't take it as advice - I am amazed and disgusted how invasive insurers are in UK and how much goverment allows them go... it is none of their business..

I tend to believe that what matters is the speed rating, in my case it exceeds requirements (Y vs. W) and the load rating which is the same (91/94). For me it sounds almost like - how would insurers feel if I fit original Lexus 16inch wheels which were fitted to SE - tires on those are 205/55?! So if Lexus deemed this car safe with 205, I struggle to understand how it can become less safe on 235s.

As for why Lexus did it... it sounds reasonable to put wider tires on RWD car, adds grip on acceleration and lateral grip, the driving wheels has extra load and wear etc. So in theory if you fit 225s all around, your rears will wear quicker and at some point you will end-up with dilemma - replace only rears, or replace all 4 throwing away part worn fronts. In practice that is opposite - my fronts goes like 4k-8k miles before rears and I have dilemma same just opposite dilemma. I guess Lexus maybe assumed fully loaded car with 4 people, which woudl add extra load and wear on the rear, by I drive alone at 80% of the time and with minimal load. All in all - I understand why they did it, but from my experience 245 on IS250 are overkill. 

there is other option - I can go 235/245 maybe (just to get that wear balance between front and rear) instead of going 225/235... but wider tires would mean lower MPG... and sitting in never ending London traffic doesn't really require any extra grip.

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My IS300 wore the rears more than the front as you'd expect, although not by a massive margin. I can the point of doing it on a McLaren or an RC-F but on an IS250?  Does seem like overkill for marginal (at best) or questionable gains. 

Speed rating is another area where I bet insurers would try to wheedle out of paying, no matter my car can only achieve the 'Y' rating of the OEM tyres if it fell out of an RAF Hercules! Yet I'm reticent to knock the rating back even one to 'W', just in case. They don't mind with winters tyres though, apparently. 

We do have an Adrian Flux presence on the forum, perhaps they can advise us without one of us asking our own insurer and risk putting up our premiums!

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I reckon ... indifferent, but don't take it as advice - I am amazed and disgusted how invasive insurers are in UK and how much goverment allows them go... it is none of their business..
I tend to believe that what matters is the speed rating, in my case it exceeds requirements (Y vs. W) and the load rating which is the same (91/94).

When my 1st F-Sport was written-off it had the "wrong" tyres on the front (235/40R18) and no-one batted an eyelid.

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12 hours ago, gdh300 said:

My IS300 wore the rears more than the front as you'd expect, although not by a massive margin. I can the point of doing it on a McLaren or an RC-F but on an IS250?  Does seem like overkill for marginal (at best) or questionable gains. 

Speed rating is another area where I bet insurers would try to wheedle out of paying, no matter my car can only achieve the 'Y' rating of the OEM tyres if it fell out of an RAF Hercules! Yet I'm reticent to knock the rating back even one to 'W', just in case. They don't mind with winters tyres though, apparently. 

We do have an Adrian Flux presence on the forum, perhaps they can advise us without one of us asking our own insurer and risk putting up our premiums!

Yes I agree, for me load rating seems to be more important that speed rating. It is "a bit" silly because IS250 requires W (168MPH) - the speed car cannot even reach. More appropriate would be V (149MPH), because car can only achieve 141MPH, but even then I would not be worried as in UK one would need to be mental or on the race track to exceed 90-100MPH anyway. I would be quite happy to challenge any insurance in the court on the point that national speed limit is 70MPH, so basically any tire should be suitable unless they trying to encourage speeding (the lowest band L is rated for 75MPH). That said one will be pushed to find summer tires not rated for at least W and most are nowadays Y (186MPH!).

As for load rating which seems to be more relevant IS250 requires 91/94 on 17 rims and most tires exceed that. Quite funny again load rating on 16 inch rims is 91/91 - as if rear of the car is lighter on 16 inch rims?!

I personally think that as long as you have 2mm+ thread and you tires are not sort of "stretched" it should be fine. Even if you have something like 90 load rating that should not invalidate your insurance (90 is 600kg, 91 is 615kg). Generally speaking  most tires of certain size will have similar load rating e.g. 17 inch have rating between 89-95 +3-4 for XLs.

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