Help with Tyre sizes

[SBN]

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

[TA14]

Sounds OK to me. If it were my car I'd double check the handbook and then post on the BMW section to see if there's an issue specific to M3s; or maybe an owner's club, etc.

[B'stard Child]

SBN said:

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

That's a significant difference in rolling radius

The 40 is the height of the tyre expressed as a % of the width

so 245/40 is 98mm height

235/40 is 94mm height

Edit to add actually it's closer than I thought

Edited by B'stard Child on Monday 22 December 15:54

[SBN]

Thanks, called BMW they were kind of helpful but wouldn't couldn't recommend It. Same with Ats as well.

Thing is a 8" BMW alloy on the e90 takes a 225 as standard. This is a 8.5 so why jump up to a 245 surely a 235 would work??? But then BMW puts a 245 on for a reason and must contribute to handling, turn in etc having the 245 vs 235

[SBN]

B'stard Child said:

SBN said:

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

That's a significant difference in rolling radius

I'd be looking more for 235/45 18

Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?

[kambites]

The "45" of 235/40R18 means "40% of tyre width". So 235/40R18 means that your overall rolling radius is (18/2) inches (for the rim) plus (235*40/100) mm (for the tyre sidewall).

Obviously (235*40/100) isn't the same as (245*40/100).

[B'stard Child]

SBN said:

B'stard Child said:

SBN said:

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

That's a significant difference in rolling radius

I'd be looking more for 235/45 18

Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?

235 is width
40 is profile (or percentage of the width)
18 is rim width

total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)

[B'stard Child]

Oh and my initial guess of 235/45 18 being more suitable was wrong - maybe 225/45 18 would have been better

Most BMW handbooks have recommendations for winter tyre sizes - have you looked there?

[z06tim]

Try this link for tyre sizes vs rim sizes: http://www.tyresizecalculator.com/tyre-wheel-calcu...

You should be fine with a 235.

And this for calculating the tyre circumference:

http://www.csgnetwork.com/tiresizescalc.html

Looks like you lose an inch of circumference with the smaller 235/40, but you gain nearly 2 inches with the alternative 235/45. I would therefore stick with the 235/40, which after all is the winter wheel for your car anyway (if i'm reading your post right).

Remember a slightly narrower front tyre may mean your car transitions to understeer more readily.

[B'stard Child]

http://tire-size-conversion.com/tire-size-comparis...

Should help

[SBN]

Sounds veeeeeerrrryyy complicated so I think I will pass on these and look for something that is the same as what I have.

Had no idea that the three measurements on a tyre were not as straight forward as you'd guess them to be. Thanks for your help.

[kambites]

As someone said above, check your handbook. It wouldn't be unusual for a manufacturer to recommend a different tyre size for winter use.

[lufbramatt]

Don't forget a tyre has about 6mm of wear between new and worn out, so the 4mm difference in height mentioned above is hardly worth worrying about IMO.

[kambites]

lufbramatt said:

Don't forget a tyre has about 6mm of wear between new and worn out, so the 4mm difference in height mentioned above is hardly worth worrying about IMO.

This is true, and even that's assuming you keep your tyre pressures perfect all the time.

[TA14]

kambites said:

lufbramatt said:

Don't forget a tyre has about 6mm of wear between new and worn out, so the 4mm difference in height mentioned above is hardly worth worrying about IMO.

This is true, and even that's assuming you keep your tyre pressures perfect all the time.

Yes, as I posted above, 4mm or just over 1% won't normally bother a car, unless there is a specific M3 issue that no one has mentioned.

[SBN]

I'll have a look at the handbook later but I did speak to BMW and they would only specify the tyres I have on the car without another option being available.

[TA14]

SBN said:

I'll have a look at the handbook later but I did speak to BMW and they would only specify the tyres I have on the car without another option being available.

Predictable - they didn't say that it wasn't OK though. You'll be running with the same radius that you run when the 245s are quite worn (but still legal) and a width which won't bother you unless you're at more than 9/10s of the handling capability. Unless there's some weird electronic sensor issue or something you should be OK - that's why you could just do with a BMW expert to confirm it. You need a good independent BMW specialist or a friendly tyre company engineer that knows the car to remove that 1% of doubt; I guess that most people would just try the tyre.

[Debaser]

B'stard Child said:

SBN said:

B'stard Child said:

SBN said:

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

That's a significant difference in rolling radius

I'd be looking more for 235/45 18

Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?

235 is width
40 is profile (or percentage of the width)
18 is rim width

total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)

The 18 refers to wheel diameter in inches, not width.

Wheel width will be a number followed by the letter J.

[SBN]

TA14 said:

SBN said:

I'll have a look at the handbook later but I did speak to BMW and they would only specify the tyres I have on the car without another option being available.

Predictable - they didn't say that it wasn't OK though. You'll be running with the same radius that you run when the 245s are quite worn (but still legal) and a width which won't bother you unless you're at more than 9/10s of the handling capability. Unless there's some weird electronic sensor issue or something you should be OK - that's why you could just do with a BMW expert to confirm it. You need a good independent BMW specialist or a friendly tyre company engineer that knows the car to remove that 1% of doubt; I guess that most people would just try the tyre.

It's £300 against 1% doubt good odds but I'm not the expert. Love the help on here guys it's really useful but will instead look for the correct tyres and then I won't get any warranty issues or similar etc

[B'stard Child]

Debaser said:

B'stard Child said:

SBN said:

B'stard Child said:

SBN said:

Got a 2009 m3 with 18" alloy set up

The summer setup on the fronts (8.5 wide) is a 245 40/18. Will it be wise, ok, crazy to use a 235 40/18 as I have some for a winter set up on this rim?

That's a significant difference in rolling radius

I'd be looking more for 235/45 18

Interesting comment I thought the profile combined with diameter of the alloy attributes to rolling radius?? Am I wrong in my limited understanding as I thought the 235 is how wide not how round a tyre is?

235 is width
40 is profile (or percentage of the width)
18 is rim width

total rolling radius is a combination of tyre and rim width but you can just as easily work out the difference on just the tyre (if the rim diameter doesn't change)

The 18 refers to wheel diameter in inches, not width.

Wheel width will be a number followed by the letter J.

Correct - my error