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Discussion Starter #1
Hi All

My 4 Series 420D M Sport has OEM Bridgestone Potenza S001 Run Flat tyres.

255 x 35 x 19 on the rears, and 225 x 40 x 19 fronts

The rear tyres will need replacing soon, and I have been offered a great price on a pair of new Potenza RE050A Run Flat tyres , but these are 255 x 30 x 19 (a slightly lower profile)

Is this combination ok with the front axle, or indeed ok at all? Bearing in mind the slight profile difference, and the different tyre tread pattern.

Not sure if anyone has done the same, or could give any advice regarding this please?

Thanks :)
 

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Discussion Starter #3
You might run into ABS problems as the software could think there is a flat tyre or wheel spin with the discrepancy in sizes as it expects all tyres to have the same rolling radius
Ok, thanks for the reply, appreciated.

On further research, I see that changing any standard tyre setup to different profile size, does have an impact on speedo accuracy, and possible handling problems, especially if it is a significant change.
I came across this useful Tyre size calculator that shows the changes to speedo accuracy and tyre measurement differences, when changing to different size tyres.

https://tiresize.com/calculator/

So even though BMW use different tyre sizes on the front and rear, they select the appropriate tyre sizes to match an equal rolling radius with no detriment to handling\wear etc...

I'm still wondering though in "real world terms", if the setup I was looking at would really be a no no, as it's around minus 3.8% difference in diameter ......

Just food for thought :)
 

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I'm still wondering though in "real world terms", if the setup I was looking at would really be a no no, as it's around minus 3.8% difference in diameter ......
Well the software is setup to allow for a small discrepancy as the front tyres could be at 1.6mm and the rears could be brand new at 8mm so it just depends on what 3.8% works out to in mm and if you do fit them and it does cause a problem you have the extra expense of having to buy new tyres again
 
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