[Z3man]

Hi,

Can anyone tell me why M-Sport offset wheels have a lower profile on the rear, yet when you have square set up they are the same profile all round, yet the wheel diameters are the same.

My fronts are 225/45/17

My rears are 255/40/17

[RajB]

It's all to do with the width of tyre being used and rolling circumference.

Front to rear needs to be as close as possible so when you use a wider tyre the side wall profile needs to reduce to maintain the rolling circumference.

[IanS100]

As tyres get wider they also get taller, which is why we have aspect ratio, which is used to even up the rolling radius.

[Digitalize]

Tyre profile is a percentage of the width.

[Z3man]

Originally Posted by RajB

It's all to do with the width of tyre being used and rolling circumference.

Front to rear needs to be as close as possible so when you use a wider tyre the side wall profile needs to reduce to maintain the rolling circumference.

Thats what i thought, so why don't they fit 255/45/17, then they would have the same profile and rolling circumference as the fronts.

[MrPogle]

They wouldn't.

The height of the sidewall is the width x profile.

So 255 x 0.40 is the same as 225 x 0.45. About 102 mm.

[Z3man]

I think where i am getting confused is because i always thought the profile 40 was a set depth like 40mm, but it turns out it is not it is part of an equation. So a profile of 40 on a 255 tyre is a different depth than the profile of 40 on a 235 tyre.

Am i getting it now.

[mr_mason]

Yes mate..

The '40' is a percentage of tyre width.
225mm wide tyres, with a sidewall of 40% of that width.

So a 225/40 tyre will have a smaller sidewall size than a 235/40 tyre.

And on the 18" m sport wheels, 40% of 225 and 35% of 255 give similar size tyre walls, within 1mm I believe.

Hence why the tyres are matched for overall circumference based on the width of the wheels.

[Z3man]

Thanks for all the info, i can't believe i have been wrong for the last 30 years or so.

I have been thinking of putting slightly narrower runflat tyres on the rear because there is more choice then. Would i be ok to go with either of these 245/45/17 or 235/45/17, that would give me profiles of 110 and 105 respectively , would they be too high.

[mr_mason]

On paper there is no reason why you can't run whatever tyres you want.

The 235/45 tyre would give a 'closer to original' tyre wall size. Which would maintain the highest percentage of speedo accuracy. But would give a slightly different look to the rear tyre.

[tomwh85]

I've just learnt something new

[Z3man]

Now i know how to work it out thanks to you guys, i have been looking into it and 245/40/17 gives me a profile of 98, which is pretty close to the 101 profile of the fronts.

I don't think the speedo would be affected by what we are talking about as the fronts will be staying the same as they are now. Thats if i am right in thinking the speedo works off the front wheels.

[shaun335d]

To many clever people on here

[mr_mason]

Originally Posted by Z3man

Now i know how to work it out thanks to you guys, i have been looking into it and 245/40/17 gives me a profile of 98, which is pretty close to the 101 profile of the fronts.

I don't think the speedo would be affected by what we are talking about as the fronts will be staying the same as they are now. Thats if i am right in thinking the speedo works off the front wheels.

speedo is in the rear as i believe its taken from the speed sensor on the output shaft of the gearbox.

its only the rear wheels you need to watch for changes, its just something to think about really, you arent going to make it incredibly inaccurate by just changing the tyre profile, its when you start fitting huge wheels that it becomes a problem without having to revert to rubber band tyres.

[RajB]

Originally Posted by Z3man

Thats what i thought, so why don't they fit 255/45/17, then they would have the same profile and rolling circumference as the fronts.

As others have pointed out I should have gone into depth a little more but yes the profile is a percentage and not a designated set depth.

[Jules59]

Originally Posted by tomwh85

I've just learnt something new

play around with this website - and youll soon get the idea

http://www.kouki.co.uk/utilities/vis...ize-calculator

[R99]

Rather than start another thread, I wanted to ask some of the experts on here about tyre sizes for the 19" alloys I'm planning to order tomorrow!

The wheels will be 19x8.5 and 19x9.5 - and the 'recommended' tyre sizes for these widths seems to be 235/35/19 front and 265/30/19 rear.

I am struggling to find any decently priced tyres in these sizes, but I was looking earlier on the Toyo website and the following info is listed:

235/35/19 - overall diameter: 644mm

265/30/19 - overall diameter: 640mm

However..... when you look at the 225/35/19 tyre specs the overall diameter is a much closer match to the rear at: 641mm

Stuggling to understand why a 235/35/19 is recommended, when it runs 'fatter' when compared to a 265/30 rear??

Would it be better just to go with the BMW recommended sizes for 19" (225/35 front and 255/30 rear) and put up with the slight stretch that will give on 8.5j and 9.5j when compared to the OEM 8j and 9j rims?

[IanS100]

If you're proposing to change to non-standard tyre sizes I would consult your insurer first, just in case they're difficult.

[R99]

Originally Posted by IanS100

If you're proposing to change to non-standard tyre sizes I would consult your insurer first, just in case they're difficult.

I'm with Sky Insurance and have a modified car policy.

There wont be an issue as long as the tyres that are being fitted are within the recommended rim range

225/35/19 and 255/30/19 are both within the max width range when fitted to 8.5j and 9.5j

[mr_mason]

Originally Posted by R99

Rather than start another thread, I wanted to ask some of the experts on here about tyre sizes for the 19" alloys I'm planning to order tomorrow!

The wheels will be 19x8.5 and 19x9.5 - and the 'recommended' tyre sizes for these widths seems to be 235/35/19 front and 265/30/19 rear.

I am struggling to find any decently priced tyres in these sizes, but I was looking earlier on the Toyo website and the following info is listed:

235/35/19 - overall diameter: 644mm

265/30/19 - overall diameter: 640mm

However..... when you look at the 225/35/19 tyre specs the overall diameter is a much closer match to the rear at: 641mm

Stuggling to understand why a 235/35/19 is recommended, when it runs 'fatter' when compared to a 265/30 rear??

Would it be better just to go with the BMW recommended sizes for 19" (225/35 front and 255/30 rear) and put up with the slight stretch that will give on 8.5j and 9.5j when compared to the OEM 8j and 9j rims?

what is even more ironic is that the actual diameter for the 265/35/19 is 641.6mm, im not trying to be pedantic but it just illustrates the point even more that BMW seem to have chosen an odd size tyre for the fronts when fitting 19's.

i dont know why its been decided that the fronts should have 235/35's, it may be something to do with the width of the wheel being a 8.5" rather than just an 8" like the 18's. maybe they didnt want to be seen to be running the same size tyres on their 8" and 8.5" wide wheels when used on the front. i imagine someone in a much nicer office than mine, who gets paid a lot more than i do, made the decision and thats how it is.

when i eventually get around to changing the wheels i think i am going to go for 35 profiles front and back, would rather have a slightly oversized tyre wall on the rear than a smaller one when compared to the front.