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Replacing 245/40/18 With 235/40/18

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I apologise if this has been answered elsewhere, I cant seem to find anything.

Can you replace the stock 245/40/18's with 235/40/18's. I was looking at the KU31's and its a difference of $75 per corner?

Is there a different size that's more compatable with the stock 245/40/18?

Any advice would be greatly appreciated.

Cheers

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If you go from 245/40 to 235/40 the rolling diameter will change, so your speedo will be further out, and revs will change on the highway.

And I think not for better......

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The stock load is 93Y and the rating of the KU31 is 95. I am guessing if the replacement tyre load is the same or higher then it should be ok.

I have heard there is a rolling diameter tolerance of 15%, is that correct?

I am really curious if there is anyone who has substituted the 235's.

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you can get the tyre size changed in the computer in a few minutes, get your tuner to do it for you. Speedo will probably be more accurate if you don't though, they are all out on the + side from factory

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235's will make the diameter smaller and hence you will be doing prob 94km/h when your speedo shows 100...

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235 /40/ 18 is 645 mm OD 235/45/17 (std size on XR6) is 644mm OD so no difference. The difference between 245/40/18 and 235/40/18 is only 1.2 % so a difference of 1.2kph at 100. Speedos read 3-5 higher at 100 than your actual speed so yours will just be more correct. JUST BUY THEM.

Edited by arronm

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235's will make the diameter smaller and hence you will be doing prob 94km/h when your speedo shows 100...

235's will make the diameter smaller and hence you will be doing prob 94km/h when your speedo shows 100...

I think you calculator is broken again.

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ive been using 235/40r18 in dunlop 3000a for the last 3 years because of the price difference, have done about 70000kms without any problems the only issue is that the narrower tyre provides less grip, and the diameter is the same so it wont affect your speedo

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The diameter is NOT the same.

It is simple maths. 235/40 means the sidewall height is the tyre width times 40% or 235x0.4, which quite clearly is not equal to 245x0.4.

The difference in rolling circumference is 24.12mm and when your speedo shows 100km/h you will be doing roughly 98km/h.

Edited by winstor

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5 years later and I'm still confused over why Ford did not choose to stick with the 235/40/18 when it's diameter is effectively the same as a 235/45/17 thus no need to change the computer. I think it was a ploy to sell 245/40/18 tyres by Dunslop and make people pay extra $$$'s for a tyre size only Ford use. I suppose Ford and the girls think 1 cm extra makes a big difference lol. I think the VE did the same and uses 245/45/18 tyres.

 

Anyway has anyone used the Achillies 123S street legal semi slicks before and will 265/35/18 fit on the FG Snowflake rims on a BA turbo sedan. Only ask cos they are dirt cheap for me.

 

Cheers.

Achillies 123S Semi Slicks.jpg

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I use them. Great for the price they are dirt cheap. Plenty of grip in the dry. Will wear fast and are very noisy probably because of the tread pattern

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Sorry to bump an old thread.

 

I recently purchased a BAII XR6T Magnet ute with the 18 inch rims.  The tyre placard says 245/40/18, the tyres fitted were 235/40/18.  

 

I didn't think too much of it, but I then acquired another set of rims and tyres, but the tyres were 245/40.

 

When I stacked the 'old' rims on top of one another, the rim edges bumped against each other - meaning they were 'exposed' the most - the tyres sat 'inbound' of the rim edge if that makes sense.  When I stacked the 'new' rims (with 245/40) on top of one another, the tyres rested on each other, so the rim edges were sort of 'protected' as the tyres sat out further than the rim.

 

It probably doesn't seem like much, but I can envisage far more rim damage with minor 'kerb incursions' with the 235's than the 245's.

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