Anyone know a bit about tyres?

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Canary_Yellow

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According to the manufacturer, I should have 215/40R18's on the front of my car.

Problem is, I can't find anywhere that stocks these in a runflat.

I've got a flat front right (pot hole) so will likely need a new tyre.

Any reason why I can't put a 225/40R18 on instead? I realise I'd have to replace both front tyres if I did this, but they're coming up for 2 years and 20k miles so I'm not too worried about that.
 
Shouldnt be a problem, however belt and braces you should advise your insurance co! Crazy i know.

What car is it? every car with run flats i have had has been awful (noisy, hard, harsh ride, expensive). I ditch them as soon as possible and buy a gel and compressor set in case of a puncture.
 
I'm with Rooter. Run flats make the car noisy and you feel every stone you drive over. You can change. They claim your car is set up suspension wise to go with run flats but unless you are driving on the edge I doubt you would notice the difference.

Another vote for talking to your insurance company. Never give them an excuse.
 
Rooter said:
Shouldnt be a problem, however belt and braces you should advise your insurance co! Crazy i know.

What car is it? every car with run flats i have had has been awful (noisy, hard, harsh ride, expensive). I ditch them as soon as possible and buy a gel and compressor set in case of a puncture.
Click to expand...

Thanks Rooter and Lord Tyrion. Is the need to advise the insurance company because it's not the manufacturer recommended tyre?

It's a BMW 120d M Sport - I had considered going to non run flats, however as I'm planning to switch the car towards the end of the year I think I'll need them on there when it comes to resale/trade-in.

At least the price of them has come down by c. £50 per tyre since I had a full set done in 2013!
 
It is because it is a change from the standard, how the car was produced. When you change tyres normally you don't have to stick with the same brand, the restrictions are not that tight. What you are doing though is changing from the mfrs recommendations. Most insurers will just make a note, no charge and you move on. However, if you had not informed them and you tried to make a claim then that would be the chink of light they need to avoid paying out.

I advise my insurer each year that I am putting winter tyres on. Crazy but it is best to be on the safe side.
 
GreiginFife said:
What width are the rear tyres? Not sure I would have wider front tyres than at the rear TBH. Other way round on a RWD possibly but any change can affect the driving characteristics.
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M sport, i will wager they are 245. probably on 261M wheels. My inner BMW geek is coming out!
 
Thanks GreiginFife, it's RWD and the rears are 245/35R18, so that shouldn't be a problem.

Lord Tyrion - I doubt they'll mind but you're right that I should let them know. All I'm doing is moving to a tyre that is 1cm wider but as you say, why take the chance?

It's really annoying that the size I need aren't readily available. BMW seem to have chosen a somewhat non-standard tyre size as the manufacturer's recommendation!
 
I used to have run flats. I now don't and I have a less sore back. Some garages insist you get them but obviously they will as they are more expensive.
 
Region3 said:
Do modern cars still use rpm's of the wheels to determine speed, or am I stuck in the 70's?
Click to expand...

You are most likely correct (some new ones may use a new method), but by changing the width of the tyre, you are in turn changing the diameter potentially making the speedometer incorrect. not by much in this case, probably sub 1-2%.
 
Lots of people run smaller tyres in winter compared to summer but as long as the difference is small you will be in the territory mentioned by Rooter, 1-2%. If you have a satnav then you can check your speedo against that as the satnav is likely to be more accurate. 73 on my speedo is actually 70mph on my satnav for example. Check that and then you can gauge whether you need to get your speedo re-calibrated or, more likely, you just work out the difference and live with it. I suspect the difference will be small and no different to someone driving with under inflated tyres compared to over inflated tyres.
 
Rooter said:
You are most likely correct (some new ones may use a new method), but by changing the width of the tyre, you are in turn changing the diameter potentially making the speedometer incorrect. not by much in this case, probably sub 1-2%.
Click to expand...

Why does changing the width change the diameter? EDIT: just looked it up, it's to do with aspect ratio element, so to maintain the same diameter I would need to reduce the aspect ratio to a 35? EDIT NUMBER 2: No - it seems that wouldn't solve the problem http://www.sizemytires.com/calculators/compare - I can live with a 1.26% difference though so that should be fine.

I spoke to my local BMW dealer, they were very helpful and advised that Bridgestone no longer make the manufacturer recommended tyre for my car but that there is no issue with putting the 225's on it instead.
 
Last edited:
Canary_Yellow said:
Why does changing the width change the diameter? EDIT: just looked it up, it's to do with aspect ratio element, so to maintain the same diameter I would need to reduce the aspect ratio to a 35?

I spoke to my local BMW dealer, they were very helpful and advised that Bridgestone no longer make the manufacturer recommended tyre for my car but that there is no issue with putting the 225's on it instead.
Click to expand...

going from 215 to 225 increases the width, yeh... ok, the 40 part of 215/40/18 is actually the % of the width, so:

a 215/40/18 has a side wall height of 86mm
a 225/40/18 has a side wall height of 90mm

So you have increased the overall diameter by 8mm.
 
Increased the diameter by 8mm, but increased the circumference about 25mm.

215/40 would be 540mm circumference, and 225/40 would be 565, an increase of 5%. If the speedo does depend on this it would read under speed which isn't ideal with all the speed cameras about.

Maybe BMW's are clevererererer, or they use the rear drive wheels for speedo?
 
Rooter said:
going from 215 to 225 increases the width, yeh... ok, the 40 part of 215/40/18 is actually the % of the width, so:

a 215/40/18 has a side wall height of 86mm
a 225/40/18 has a side wall height of 90mm

So you have increased the overall diameter by 8mm.
Click to expand...

Thanks! I was just running the numbers (it's my inner geek) and calculated it back to be a 1.27% change - the same as the website. I don't think it's possible to get closer to the overall circumference with any other combination - a 225/35R18 would be 14.5mm smaller in diameter.
 
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