Tyre widths

[snow_muncher]

I thought I vaguely understood tyre sizes, but how does 200/640R17 work out ?

( its a size listed for Kumho TWO1s' )

[giandougl]

Width x height x wheel rim radius/ diameter size, is the norm.

205mm width of tyre x 45mm height of tyre from wheel rim edge x R16 wheel radius/ diameter 16 inches/ (407mm). (As fitted to Fabia VRS MK1)

(Will be corrected no doubt with a better knowledgeable member.)

200 x 640 x R17

200mm width x 640mm height x R17 (432mm) wheel radius/ diameter.

[snow_muncher]

Thats what I thought, but 640mm wall height seems huge ?

[Gizmo]

I thought I vaguely understood tyre sizes, but how does 200/640R17 work out ?

It's simply because most competition slicks (including moulded tarmac rally tyres) are imperial measurements as they are cross-ply's whereas road tyres are generally radials and metric sizes.

[snow_muncher]

So what unit sizes are being used ~ .640 inches ?

i.e. the stated 200/640R17 = mm/inches/inches

[softscoop]

Width x height x wheel rim radius/ diameter size, is the norm.

205mm width of tyre x 45mm height of tyre from wheel rim edge x R16 (407mm) wheel radius/ diameter 16 inches/.(As fitted to Fabia VRS MK1)

(Will be corrected no doubt with a better knowledgeable member.)

200 x 640 x R17

200mm width x 640mm height x R17 (432mm) wheel radius/ diameter.

Sorry but that's wrong.

205 is the width in mm, 45 is the profile as a percentage of the width of the tyre, and 16 is obviously the diameter of the rim.

so a 205/45/16 is 205mm wide, 92.25mm profile and 16" inner rim.

[Gizmo]

Having read what I just wrote cross-plys must be metric not imperial?

The 640 should be IIRC mm, which actually makes the sidewall smaller than on a 225/40 tyre

[snow_muncher]

I think I've found my answer, the '640' is in mm, and is the overall diameter of the tyre

So 200/640R17 is pretty close to a 225/45R17 (634mm) or a 225/40R18 (637mm)

Kumho info

kouki size calculator

tyretrack info

[Llanigraham]

I presume you realise those tyres are not road legal, unless you are taking part in an MSA and/or FIA registered event?

[snow_muncher]

I was just investigating really. Wondered how the motorsport tyre sizing system worked. Seems a lot more logical than the system for road tyres.

I would love to get a set of such tyres onto the car and see what difference they make though

[Gizmo]

I would love to get a set of such tyres onto the car and see what difference they make though

Proper slicks / moulded slicks (tarmac tyres) make an unbelievable difference over even the very best road tyres, however something has to give and in this instance it is tyre wear, if you get 100 miles out of the front tyres on a FWD car then you have done well!

I have worn a brand new pair of Michelin slicks down to the canvass in less than 50 miles and at (IIRC) £120 a corner it's not a cheap game

[snow_muncher]

tbh it was more the wet tarmac and forest/gravel tyres I'd really like to try out ( that's what Devon roads are like )

[KenONeill]

That sounds like a competition tyre, in which case 200mm tread width, 640mm overall diameter, to fit a 17" wheel. As above; not strictly road legal unless the car is used regularly in competition.

[ril3k]

will a tyre size 245/40 R18 fit a superb combi?

what impact will that cause apart from better road holding?

[d.richards2]

Sorry but that's wrong.

205 is the width in mm, 45 is the profile as a percentage of the width of the tyre, and 16 is obviously the diameter of the rim.

so a 205/45/16 is 205mm wide, 92.25mm profile and 16" inner rim.

I would second this.

Also, i think putting any metric diameter tyre on an imperial rim is not road legal in the uk.

[d.richards2]

will a tyre size 245/40 R18 fit a superb combi?

what impact will that cause apart from better road holding?

I am not sure what the original tyre size is but general rules are;

Lower profile = better road holding, improved steering feel but less comfort and more noise. If you change the overall radius of the wheel with tyre, the sppedo will be affected.

Larger width = more grip but also more road noise and higher risk of aqua planing. As the profile (you are quoting 40) is a percentage of the width, if you make a change to the width, you need to assess whether the profile is still correct. There is obviously a limit to the range of tyre widths you can use without changing your wheel.

If you make a big increase to the tyre width of overal diameter you may have problems with it catching on the arch - i am sure some one on here can tell you how big you can go without needing to worry though

Hope this helps

[ril3k]

I am not sure what the original tyre size is but general rules are;

Lower profile = better road holding, improved steering feel but less comfort and more noise. If you change the overall radius of the wheel with tyre, the sppedo will be affected.

Larger width = more grip but also more road noise and higher risk of aqua planing. As the profile (you are quoting 40) is a percentage of the width, if you make a change to the width, you need to assess whether the profile is still correct. There is obviously a limit to the range of tyre widths you can use without changing your wheel.

If you make a big increase to the tyre width of overal diameter you may have problems with it catching on the arch - i am sure some one on here can tell you how big you can go without needing to worry though

Hope this helps

hi

thanks for the feedback. the stock rims are 225/40 R18. 245/40 R18 represents a average 2.5% increase based on various online computations/formulas i found. 2.5% appears to be the maximum recommended change. i read such a change will result in a speedo understatement of 2.5% i.e. i'm actually at a faster speed by 2.5% than what i see on the speedo

the question i have is do i still follow the tyre pressures for the car or do i slightly over inflate it? how does the change from 225/40 R18 to 245/40 R18 impact recommended tyre pressure?

[mannyo]

245 will not fit onto the original wheels as they are quiet narrow at only 7.5 inches wide, you would need a wider wheel to fit 245. 235 just about work on a 8 inch wide wheel, so you'll need something around 8.5J in width.

Speedo difference may only be 2.5%, but the overall tyre radius is 1/2" bigger, result will be a tyre thats going to rub the arches a lot and look quiet silly in the wheel arch. You really need 245/35 x 18 tyres to maintain the correct radius.

[vRS_Pagey]

Totally digressing but @ snowmuncher - I love your avatar.

[ril3k]

245 will not fit onto the original wheels as they are quiet narrow at only 7.5 inches wide, you would need a wider wheel to fit 245. 235 just about work on a 8 inch wide wheel, so you'll need something around 8.5J in width.

Speedo difference may only be 2.5%, but the overall tyre radius is 1/2" bigger, result will be a tyre thats going to rub the arches a lot and look quiet silly in the wheel arch. You really need 245/35 x 18 tyres to maintain the correct radius.

hi i'm not on the original rims. somehow my mechanic can fit the 245/40 R18 on and it does not seem to rub the arches

i took some pics but somehow cant load it on

[ril3k]

My link

view of wheel

Edited 26 August, 2011 by ril3k