How To Calculate Rolling Radius

Calculating your rolling radius isn’t so bad as long as you do it in stages. The main problem is that for reasons of convention, wheel height is measured in inches whereas tyre profile (height) is expressed as a percentage of the section width – which is measured in millimetres! Below, we’ve written out the steps you need to go through if you want to calculate your rolling radius:

Find section width

This will be printed on the side of your tyre and will typically be three digits long. In the photo above, the section width is ‘225’ – meaning that the tyres are 225 mm wide.

Find aspect ratio

This will generally be the next figure in the size printed on the side of your tyre. In the photo above, the aspect ratio is ‘40’ – meaning that the sidewall height is equal to 40% of the section width.

Find rim size

This will generally be the next number in your tyre size – and probably comes after an ‘R’ (which stands for ‘radial’). In the photo above the rim size is ‘18’ – meaning that the tyre is designed to be mounted on an 18 inch rim.

Calculate sidewall height

For this, you need your section width and aspect ratio figures. Take your aspect ratio (in this case 40) and add a zero and decimal point before it (0.40 in this case). Multiply your section width by this figure to give the sidewall height and make a note of the result. For the photo above, the sidewall height is 90 mm.
225 x 0.40 = 90

Convert rim size to mm

We already have the rim size, but it’s measured in inches rather than mm. To convert inches to mm you just need to multiply by the magic number (25.4).
For the photo above, the rim size in mm is 457.2 mm.
18 x 25.4 = 457.2

Divide rim size in mm by two

We’re interested in the radius of the wheelset – not the diameter – so we only need half of the rim size. Divide the mm figure you just calculated by two.
For the photo above, half of the rim size in mm is 228.6 mm.
457 / 2 = 228.6

Add this figure to the sidewall height

Now that we know half the height of the rim in mm, we can add it to the sidewall height to find the overall rolling radius. We only need to worry about the sidewall height once because we’re calculating the radius of the wheelset – not the diameter.
For the photo above, the overall rolling radius is 318.6 mm.
228.6 + 90 = 318.6

If you do this for both your existing and new setups, you will be able to see the difference (if any) that they will give in terms of rolling radius. As long as your old and new measurements are within about 2.0% of each other, you shouldn’t run into any problems.

Tyre Width

So, now that you know all about a wheelset’s vertical dimension, what about the horizontal? A set of wide tyres will look great (especially from behind the vehicle), but go too wide and you could risk messing up your car’s handling or fuel consumption – or the wheelset simply not fitting under your arches! So this section explains everything you should know about tyre width.

Tyre width is a bit more complicated than just knowing your rim width and matching a tyre to it. Because of the way a tyre mounts on the rim, you can usually oversize or undersize them a bit – which can be helpful in certain situations. The thing is, the amount you can do this varies with the profile of the tyre – because there will be more or less sidewall to stretch and accommodate any size difference. Sometimes there will be other factors at play when choosing tyres that we can’t account for here – so if you’re at all unsure, then please give our Wheels and Tyres Team a shout and they will give you a definitive answer.

Rim Width (inch)	Min. Tyre Width (mm)	Ideal Tyre Width (mm)	Max. Tyre Width (mm)
5	155	165 or 175	185
5.5	165	175 or 185	195
6	175	185 or 195	205
6.5	185	195 or 205	215
7	197	205 or 215	225
7.5	205	215 or 225	235
8	215	225 or 235	245
8.5	225	235 or 245	255
9	235	245 or 255	265
9.5	245	255 or 265	275
10	255	265 or 275	285
10.5	265	275 or 285	295
11	275	285 or 295	305
11.5	285	295 or 305	315
12	295	305 or 315	325
12.5	305	315 or 325	335