Tyre sizes

Greg

Greg

Greg

Soldato
Joined
18 Oct 2002
Posts
8,973
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
 
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.
 
As above tbh, because you have such a difference in the width of front v's rear anyway, BUT at the end fo the day the car was designed to work with 40/35 profile split and now it's not got that. Call them up and ask to swap them and while at it, ask to add additional for a decent set?
 
maccapacca said:
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.
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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?!
 
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!
 
Just a thought but you wouldn't want to spin it ditch it and have an insurance company crawl all over it and declare you had the wrong tyres on it, might be miniscule in the way of risk but if you slid and killed someone they'll pass the buck faster than hot coal.
 
They are definately the wrong size - your car should have 255/35/18's on the back. I do hate it when garages do this - I'd rather they sold the car with the old tyres and gave a discount equivilent to what they would spend on new tyres - you don't want crap tyres on the back of a light, tailhappy 270bhp sports car. You want decent tyres (No, not Falken).
 
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!
 
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.
 
Regarding garages putting rubbish tyres on cars, I had a similar thing when I bought my car. The garage proudly told me "we've put a new tyre on it!" - I checked the tyre in a question to find it was a brand new Nexen, on the other side was a UHP tyre about 90% of the way through its life. On the front were near-as-dammit new Avon ZZ3s (although the salesman had no idea, good knowledge of the car....) There was no point them even having bothered with the Nexen, when I got home with the car I had two new CSC5s waiting for it in the garage.

The tyre fitters were confused when I told them to take off the brand new tyre and do with it what they will. Doesn't surprise me really, it's probably the sort of tat they peddle to customers all the time.
 
[TW]Fox;24123163 said:
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.
Click to expand...

You're right; all in good time! :)
 
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