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Hi guys,

I have stock 17" tyres which are a staggered configuration of: 225/45/17 and 245/45/17. I want to put on some 18" wheels from the new generation IS and the tyre size is: 225/40/18 and 255/35/18.

I asked a tyre shop and they said "225/40/18 matches the height of a 225/45/17 so the tyre you have for the front (225/45/18) is too tall. Are you sure your current rear tyres are a 255/45/17? I would have guessed 255/40/17, in which case a 255/35/18 is ideal."

A few shops have said that the tyre size on the stock 17" wheels is a bit strange...

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Sorry, you were right. The 18" rear was 35 profile sorry, edited initial post.

Stock size is: 225/45/17 and 245/45/17
New size is: 225/40/18 and 245/35/18

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I've got 255/35/19 on my 19" so I imagine it wouldn't be a problem

You could try 225/40/18 on fronts and 255/40/18 on the rears. They are standard tyre size and should not be a problem.

245/35/18 should not be a problem either unless someone can chime in for me

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Sorry, you were right. The 18" rear was 35 profile sorry, edited initial post.

Stock size is: 225/45/17 and 245/45/17

New size is: 225/40/18 and 245/35/18

It appears that QLD allows a max of 15mm dia difference from original spec.

For your front it's 2.9 which is ok, but rears come out to be -23.6 - I'm not sure this is legal...

Are you sure the spec is 245/35 for rear and not 255/35 or 245/40 ?

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the rears are 255/40/18 on my car. factory 18" rims

the factory size for the rear 17" is 245/45/17

Yes, between these 2 - it's only 8.9mm which would appear legal.

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i just checked both 2IS and 3IS, it appears the SL models share the same tyre specs @ 255/40/18, but the 3IS FSport does run 255/35/18.

not sure why this is the case.

given OP car is a 2IS, 255/40/18 would be the ideal size to run, as per factory specs. assuming wheel sizes are the same.

as strange as the specs are for the rears, they are factory sizes.

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Sorry, you were right. The 18" rear was 35 profile sorry, edited initial post.

Stock size is: 225/45/17 and 245/45/17

New size is: 225/40/18 and 245/35/18

It appears that QLD allows a max of 15mm dia difference from original spec.

For your front it's 2.9 which is ok, but rears come out to be -23.6 - I'm not sure this is legal...

Are you sure the spec is 245/35 for rear and not 255/35 or 245/40 ?

Oh wait, it's 255/35/18 for the new rears. Keep getting the sizes mixed up - would it be within the limits now?

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QLD rules are:

The rolling diameter cannot increase more than 15mm and cannot decrease more than 26mm.

Seeing as it's a reduction in rolling diameter it should be fine, is that right?

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Sorry, you were right. The 18" rear was 35 profile sorry, edited initial post.

Stock size is: 225/45/17 and 245/45/17

New size is: 225/40/18 and 245/35/18

It appears that QLD allows a max of 15mm dia difference from original spec.

For your front it's 2.9 which is ok, but rears come out to be -23.6 - I'm not sure this is legal...

Are you sure the spec is 245/35 for rear and not 255/35 or 245/40 ?

Oh wait, it's 255/35/18 for the new rears. Keep getting the sizes mixed up - would it be within the limits now?

255/35 is 16.6mm, according to ICU2 , he found 26 mm is acceptable for a decrease. I guess you're good to go.1 Thing to remember is that your speedo value will be off by approx 3Kph - Higher... So at 100 you'll actually be travelling at 102.54Kph.

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1 Thing to remember is that your speedo value will be off by approx 3Kph - Higher... So at 100 you'll actually be travelling at 102.54Kph.

Fortunately, there's a reasonable difference between speedo reading and actual speed when standard wheels and tyres are fitted; so at 100, you could well be travelling below 100, even with the new wheels fitted. But you'll have a much smaller margin, so be careful around speed traps!

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Yep, I'll see what speed the GPS is going. Will this also have an effect on the km reading of my car too? (eg. will it read 1000k earlier)?

GPS isn't terribly accurate. There's actually a deliberate error introduced on GPS signals to prevent it being used for military purposes. I believe it's between 4-8 meters. Indicated speed will not be accurate down to few kph.

The speedo is usually driven by the front wheels. Over a 1000k, shows up as a difference of <5km.

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Thanks, noted - I'll see if I can go past a road with one of those speed signs with the smiley faces.

Does anyone know the speed rating on the stock 17" tyres?

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You will find the tyre speed rating and size in your cars placard.

For your info my IS350 has 225x40x18" front and 255x35x18" rear , identical to my old BM335I.

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I CU2, you already have the standard 17" tyre, so have a look at the last letter after the tyre size, e.g.

225/45R17 95W

Then look up the letter (e.g. W) against this chart of tyre speed ratings:

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=35

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  • 3 weeks later...

I have a 2007 is250. I currently have 225/40/18 on the front and 255/40/18 on the rear. Thinking of changing the rears to 255/35/18, would anyone know if there is an issue with this?

Also, currently the front tyre doesn't seem to be flush with the rim.. tyres look like they're a bit stretched. Always been like that since I bought the wheels privately. Took it to Bob Jane T-Mart recently and they say I might have the rims back to front. The guy said I might have 8" wide rims on the front and 7" wide on the back, hence the front not being flush. Anyone familiar with this?

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I have a 2007 is250. I currently have 225/40/18 on the front and 255/40/18 on the rear. Thinking of changing the rears to 255/35/18, would anyone know if there is an issue with this?

Also, currently the front tyre doesn't seem to be flush with the rim.. tyres look like they're a bit stretched. Always been like that since I bought the wheels privately. Took it to Bob Jane T-Mart recently and they say I might have the rims back to front. The guy said I might have 8" wide rims on the front and 7" wide on the back, hence the front not being flush. Anyone familiar with this?

Personally i cant see the point in changing your rears from 40 to 35 profile.Benefits would be minimal.

As for the fronts looking stretched i have the same tyre/rim setup.They are supposed to be like this.The guy from Bob Jane is clueless.You could put 235/40s on the front to get rid of the stretched look and better rim protection,not sure if you'll get any rubbing though.I'm going to look into this when i'm up for front tyres.

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