Low Profile Tyres

Then if you change the tyre profile you put up with an incorrect speedo, wrong gearing and possible safety and insurance issues. Larger rims are fitted by people who want low profile tyres for performance purposes and are not suitable for higher profile tyres of the same diameter. But there's plenty people who will sell and fit them for you if you like.
 
If you look at the website linked it actually calculates for you how much inaccuraccy you are building in to your speedo. Its simple mathematics and there is a right size. The smaller the deviation the smaller the problem for sure but I look at some of the monstrosities on the road and somebody sold and fitted tyres which are blatantly incorrect on these vehicles.

The owners manual will list the suitable tyre/ rim combinations and if you calculate the "Effective finished wheel diameter" you will see they are all in a fairly narrow range with rim diameters compensating for profile changes.

1 + 1 must equal 2 at some stage.
 
HighFlier said:
The owners manual will list the suitable tyre/ rim combinations and if you calculate the "Effective finished wheel diameter" you will see they are all in a fairly narrow range with rim diameters compensating for profile changes.
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Good point!
 
Lads - I must say I am very impressed ye have had a 2 page discussion on my post - sorry I wasn't around to contribute until now. For the record my tyres are

225 45R 17 91V (Bridgestone) - note they are Run Flat tyres !!

alloys are Star Spoke 158 8J x 17

I need to re-read everyone's comments again - but I think it looks like I could go for a 16 alloy and a suitably matched tyre to maintain total wheel diameter. It seems other tyre sizes with the same alloy seem to create problems unless I misread.

A final point is that although the issue is what is known as 'tramlining' part of the less than ideal drive quality is also the hardness of run flat tyres which BMW have unfortunatley implemented.

Thanks to everyone!!
 
Lakeview said:
A final point is that although the issue is what is known as 'tramlining' part of the less than ideal drive quality is also the hardness of run flat tyres which BMW have unfortunatley implemented.
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Ah thats a new development and not an uncommon problem, i know of three people who had run-flats issued as standard and quickly replaced them due to the problems you described and a very harsh ride quality.

You may not have to downsize in this case. I'm also running 17" with exactly the same tyre size but not run-flats. I also drive a 3-series but it's the last generation (E46) not the current model.

Have a look around the bmw boards i mentioned and you'll be able to solicit more advice on the matter. I'm not sure if your local tyre shop would accomadate 'trying out' non-run-flats before deciding to downsize to 16" alloys but if not i can recommend a good tyre place on the Northside who would possibly do so as a favour (I'll ask tomorrow).

Also if you do decide to go down the road of switching alloys sizes 16" are more commonly available second hand (again on the forums) and 17" should sell on quite easily.
 
Thanks Eurofan - I would love to go to non run flats and carry a spare BUT !! the boot design does not accommodate a spare anymore in the 2005 model. I think the whole thing is stupid and this may be my 3rd and last BMW.
 
It's quite some time since i've carried a spare as well to be honest, it's bucketing down at the moment so i can't go out and check the name of it but i always carry two cans of tyre sealant which is a darn site quicker and less messy than changing a wheel. No need to worry about the spare.
 
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