Wheel + Tyre sizes - confused !

[dibbly_dobbler]

Hi All. Need your help as I have become confused. I am running a 130i se on 17 inch wheels and am thinking of upgrading to 18s. The 18s typically have a lower profile so does that mean that the overall circumference of wheel and tyre would be the same ? If so then how is it that they appear to fill the wheel arch more fully - and if not then does it muck up the speedometer/odometer readings ? Clearly I have got the wrong end of the stick somewhere
Cheers, Mike.

[martinmac]

http://reviews.ebay.co.uk/Tyres-explained-low-prof...

Any help.

[Kinky]

Everythng you need (and more)is right here: http://www.carbibles.com/tyre_bible.html

Extremely useful tyre size calculator 1/3rd down page 2

K

[dibbly_dobbler]

Thanks guys Appreciate the help. 'Er indoors now thinks I'm a complete for getting into all this but nothing new there ! If I read it right then going for a bigger wheel normally means a lower profile in order to maintain 'rolling radius' ( check me out with the terminology !). But if you really 'go large' then your odometer and speedometer will over-read but not by a lot.
So I guess if I upgrade my 17s to 18s (assuming standard tyres) then they wont actually fill the arch any more.

[paoloh]

Physically, no they will not fill your arches more but visibly, they will. Less tyre, more wheel make them look like they fill your arches better.

[dibbly_dobbler]

paoloh said:

Physically, no they will not fill your arches more but visibly, they will. Less tyre, more wheel make them look like they fill your arches better.

Thanks Paoloh. Makes sense I suppose - I had read on other sites about people needing their arches 'rolled' having gone up a wheel size or two (ie to 19s on a 1 series which apparently is a bit dodgy) hence my confusion.

[paoloh]

I've had 20's on a 3 series with no mods needed if that helps

[dibbly_dobbler]

Cheers Paolo - suspect 18s would be my limit anyway as the ride is already brick hard ! Have just checked my tread depths and the 17s have got 5-6mm left after over 16k. Should be happy I suppose but it makes it harder to justify the new wheels

(Checked out your profile by the way and love the M5... )