Calculating Tire Diameter (Imperial & Metric)
We will focus only on calculating Tyre diameter here. I will also explain with different examples, so as you will clearly digest this concept. This may look complicated, but trust me, it is easier than it looks. Calculating any Tire diameter requires a basic understanding of tire markings. I will focus only on those specifications that we actually need.
Before we proceed, don’t worry if your tire is metric or inch based, I have covered both.
First, I will cover metric ones. Because they comprise most of the Motorcycle and Car markets around the world.
Reading Tire Markings
It will be either Metric or Imperial. Metric Looks like this,
(See the picture at the top)
Metric: 165/80-14 (Ignore the rest)
Here, the first number 165 is the Width. This means the tire is 165 mm wide.
After the slash, we have the height. It will always be mentioned in Percentages. Here 80 denotes 80%. i.e. 80 percent of the Width ( First number) is the Height.
This implies, 80% of 165, i.e. 165 x 0.8, 132 mm is the Height.
The last number, 14 is the Rim Size, which is the diameter of the Rim/ Alloy. It will always be mentioned in inches in both cases, Imperial and Metric.
Now, see the below picture. Just find whether you need tire Width again to find diameter?
No. You don’t need Tire Width again. You only need linear distance here. Just by looking at the picture, you already found the diameter. That will be exactly the sum of Rim size and twice the Height.
But, here is a catch. The Rim Size is in inches. Not in mm. So you convert Tire Height to inches and then add them.
So, let’s find the diameter for the above Tire, 165/ 80-14.
You already found the value of Tire Height. 132 mm.
So summing up, Rim Size + 2 x Height,
Diameter= 14 + 2 x 5.2 = 24.4 inches. If you are wondering what 5.2 is, 132mm converted to inches.
Hence the final Diameter we got is 24.4 inches. Now you can convert it into metric if you want. 24.2 inches equates to 0.6197 meters.
Now let’s talk about, imperial. (Non-metric Fanboys). Markings look like this:
Imperial: 3.5-19 OR 3.5/3.5-19
The concept is 99% similar to Metric ones, except it is written in inches.
Here, 3.5 is Width in inches. (let it be as it is. Don’t convert it to mm)
Now, for Ratio, you may have a second number or not after the slash. If you have a second number, it is the Height mentioned directly (Not Ratio). If you don’t have a second number, that means, Width = Height. In this case, 3.5 is the Height of the Tire in inches.
Here too, the Tire Diameter is the sum of Rim Size and twice the Height. In this case, your calculation gets easy. You need not find the Height using the ratio. It is directly mentioned.
So summing up, Rim Size + 2 x Height,
Diameter= 19 + 2 x 3.5 = 26 inches.
Finally if converted, 23 inches equates 0.66 meters
Here is a direct formula, In both the below formulas, the obtained value will be in inches.
Tire Diameter (metric) = [(Width x Ratio)/ 1270] + Rim
Tire Diameter (Imperial) = [ 2 x Height ] + Rim
Now, if you want to make these calculations more complex, let us convert imperial type markings to metric type.
Lets consider an example, 3.25/3.00-17.
3.25 is in inches. If multiplied by 25.4mm, it will be 82.55mm. This is the width. The second number after slash (if available) is the Ratio. We need to calculate, what %age of 3.25 is 3.00? This will be (3.00/3.25)x 100. i.e. 92.3%. If there is no second number after slash, Width itself is the Height. i.e. Ratio = 100%
Hence in metric, 3.25/3.00-17 can be written 82.55/92.3-17. Now again if you use the Metric Formula, it will also provide you the same answer. Lets check,
Metric Tire Diameter= [(Width x Ratio)/ 1270] + Rim
Diameter= [(82.55 x 92.3)/ 1270] + 17 = 23 inches.
Imperial Tire Diameter=(2 x Height)+ Rim
Diameter= 2 x 3 + 17 = 23 inches.
Both the answers match. And this is how you find a matching metric tire for your old motorcycle.
Below are some examples of Standard Tire Sizes.
1. Maruti Suzuki Swift 185/ 65 R15
Diameter= (185 x 65)/ 1270 + 15 = 24.46 in ( 0.6212 m)
2. Honda Activa 90/90 R12
Diameter= (90 x 90)/1270 + 12 = 18.378 in ( 0.46 m)
3. Hero Honda Splendor 2.75 – 18
Diameter= (2 x 2.75) + 18 = 23.5 in ( 0.6 m)
4. Tata 407, 1109, 1010 7.5 – 16
Diameter= ( 2 x 7.5) + 16 = 31 in ( 0.78 m)
5. Yamaha FZ 140/60 R17
Diameter= ( 140 x 60)/ 1270 + 17 = 23.61 in ( 0.6 m)
6. TVS Apache 200 4V 130/70 R17
Diameter= ( 130 x 70)/ 1270 + 17 = 24.165 in ( 0.61 m)
7. KTM Duke 390 150/60 R17
Diameter= ( 150 x 60)/ 1270 + 17 = 24.08 in ( 0.61 m)
8. Mahindra Thar CRDe 235/70 R16
Diameter= ( 235 x 70)/ 1270 + 16 = 28.95 in ( 0.73 m)
9. Eicher Terra 16 10.00-20
Diameter= ( 2 x 10 ) + 16 = 36 in ( 0.91 m)
10. Royal Enfield Classic 110/90 R18
Diameter= ( 110 x 90)/ 1270 + 18 = 25.8 in ( 0.65 m)
11. Bajaj Pulsar 180/220 , 120/80 R17
Diameter= ( 120 x 80)/ 1270 + 17 = 24.56 in ( 0.62 m)
12. Aprilia SR 160, 120/70 R14
Diameter= (120 x 70)/1270 + 14 = 20.6 in ( 0.52 m)