Understanding Tyre Dimensions

[Wogan]

Hi All.

I've recently been going through the process of researching tyre dimensions to try and keep my wheel radius consistant while i get new wheels. I'm going from 16x7" rims to 18x8.5"

I thought I'd post up a little explaination of how it all works for anyone else wondering.

Take my old tyre dimensions for example -

215x50x16 where:

215 is the tyre width (looking from the front or back of the car) and is measured in millimetres,

50 is the aspect ratio of the tyre as a percentage.. ie, the height of the tyre wall is 50% of the width in this case

and 16 is the size rim it fits, measured in inches (the hole of the tyre)

.. would have been so much easier if they just did it all in inches or millimetres.....

so in order to keep the overall wheel radius the same, the following is the equation to follow:

rim radius (half the rim size) + Width * Aspect ratio

ie in my situation:

currently : 8 inches (203 millimetres) + 215*0.50 = 310.5mm

I knew i wanted 235s, so all that was left was to find the correct aspect ratio

with 18" : 9 inches (229mm) + 235*0.40 = 327mm

: 9 inches (229mm) + 235*0.35 = 311mm

Thus the tyre size i'm installing is 235x35x18

- Steve

ps. You may ask "why is this important?" ... well keeping the wheel size correct keeps your speedo accurate as well as your odometer. Also accelleration is noticably different with different wheel sizes (especially in less powerfull cars)

I'm not 100% sure either, so if someone can correct me, please do

Edited March 12, 2008 by Wogan

[cuppas]

uh

cant you just get the wheels you want, then get the dials recalibrated?

[joeyjoejoejuniorshabadoo]

bah... once you hit 180 it doesn't matter anyway

[Wogan]

uh

cant you just get the wheels you want, then get the dials recalibrated?

You could, but my intention is to keep the overall wheel size the same for performance reasons also.

[blind_elk]

You can save yourself a lot of hassle by reading the spec sheets of the tyre manufacturer. Usually 'rolling diameter / circumference' is given in the specs.

[Bullet32]

Hi All.

I've recently been going through the process of researching tyre dimensions to try and keep my wheel radius consistant while i get new wheels. I'm going from 16x7" rims to 18x8.5"

I thought I'd post up a little explaination of how it all works for anyone else wondering.

Well researched, youre mostly right but the sums that you have done will not help you when you are changing brands of tyres as no one size is the same from brand to brand.

I work for Federal Tyres and it is my job to advise people of the right fitment for their vehicle.

Tyre sizes and overall diameters are only relevant within each brand and even then they are not an exact science.

For example - A 235 / 45 R17 in one brand might measure 643mm whereas in a competing brand the measurement may be between 645 - 650mm.

There are to many variables between manufacturers to be able to guarantee that you are getting the same overall diameter as your last set.

You might have to have a look at your maths again though because a 235 / 40 R18 is a lot bigger than 327mm - your looking at about 645mm for that size.

[WazR32GTSt]

or.. you could find something that does all the work for you!

http://www.paspeedo.com/calculator.htm

Edited March 14, 2008 by WazR32GTSt

[Duncan]

very good point bullet 32.

I am amazaed at the range of actual widths and diameters in tyres that are rated at the same size. eg a falken ze326 in 265/35/18 is at least 2 cm narrower than a dunlop dz02j in 265/35/18.

Only really makes a difference in a 4wd skyline I guess, but don't mix and match brands even in the same size

[Wogan]

You might have to have a look at your maths again though because a 235 / 40 R18 is a lot bigger than 327mm - your looking at about 645mm for that size.

Actually i was talking in Radius while 645 is the diameter