Tyre sizes

[Greg]

Hey guys,

Just bought myself a BMW Z4 Coupe (07). Its such a beautiful car, however, I've one question regarding Tyres and tyre sizes.

It comes with the M Sport Double spoke 18" Alloys, the dealer I bought it from had it MOT'd before I bought it and it was given an advisory regarding the rear tyres, so they replaced them with new (albeit budget) ones. Now doing a little research, I've noticed that the ones they've stuck on the back are slightly different from what's originally spec'd.

Would someone briefly explain to me what this would do to the vehicle? I know it changes the rolling radius but is this a big issue?

Current:

Fronts: R18/225/40
Rears: R18/255/40

Original spec:


Fronts: R18/225/40
Rears: R18/255/35

What are your thoughts?

Thanks

 

[Greg]

The 35 is a %age of the width so as you haven't gone up in size the radius / diameter will be slightly smaller by 5%, how much this matters is probably sod all but it's more likely to cause an issue with the ABS or any other sensor you have, as they are budget I assume you are looking to change to something more intended for the type of car, just change them for what is required.

I calculate the rubber to be:

35's: 255*0.35 = 89.25cm
40's: 255*0.40 = 102cm

So the rolling radius would have increased slightly? Seen as they're brand new, i'm considering keeping them on for a while whilst i get used to the car, but i don't really like having budget tyres on, especially as they're the driving wheels?!

 

[Greg]

I've just plopped my Reg into black circles and its thrown out this:

our options

Please select your tyres

Front tyre size Rear tyre size
Select this option 225/40R18 W 88 Select this option 255/35R18 W 90

Maybe I should give them a call/email!

 

[Greg]

gonna email them and follow up with a phone call tomorrow lunch time.

 

[Greg]

Well, how embarrassing. This thread can be deleted into oblivion. Ive learnt today that I cannot read and remember! They are indeed the correct size and Im a complete ninny head!!!

Sorry guys!

 

[Greg]

They might be the correct size but they still sucked. You presumably bought a 3.0 Si for a reason - a reason that won't be helped with crappy no-name tyres on it.

You're right; all in good time!