One of the challenges in laying out the first course of tires was trying to get the lengths of the back wall and wings walls to match the dimensions on our drawings. While this may not sound too difficult it turns out it is a bit tougher than you might think when you don’t have a member of the Biotecture team standing next to you!

Firstly we should thank some very nice people at Morewood Tire who took the time to explain tire sizing to us (they also put us in touch with the right people to supply us with all of the tires we need, more about that in a future blog).

This information taught us how to calculate overall tire diameter which we use when we need to make up a gap between tires in a row. I even found a great WordPress plugin to create an online Tire Size Calculator.

I found the tire sizing chart above on the web for reference. While this shows what each number or letter refers to it doesn’t give you all the information you need to figure out the full diameter. There are quite a few letters and numbers on each tire but I will only detail the ones that affect the overall tire diameter.

I will use the **P215/65R15** tire shown in the image as a reference.

The first number is pretty straight forward and indicates the tire width (i.e., the part in contact with the road) in millimetres (yes, tire sizing is a nice mix of metric and imperial!). In this case the tire is **215mm** wide.

The second number refers to the aspect ratio of the tire (sometimes called the Profile of the tire). In this case the height of the sidewall 60% of the tire width (**215mm x .6 = 169mm**).

And finally the third number is the one we are all mostly familiar with, this refers to the diameter (in inches) of the rim the tire gets mounted to. In this case the tire is designed to be mounted on a **15″** rim.

The first step is to convert all of the values to the same format. In our example we convert the tread width from millimetres to inches (**215/25.4=8.46 inches**).

Now we calculate the sidewall height (**8.46 x 60% = 5.08”**).

And finally we add them all together…

Sidewall Height (top) + Wheel Diameter + Sidewall Height (bottom)

**5.08” + 15” + 5.08” = 25.16”**

Now if I am short by 4” on a row using 215/60R15 tires (25.16”) I can swap out 3 tires for 215/70R15 (26.85”). Of course given the different level of wear on each tire this is not perfect but it certainly helps when digging through the tire pile!

I hope this helps someone else out there.

Tire Size Calculator

I could have explained tire sizing to you.

Hope the earth ship is moving along. We will come and help you one of these days soon.

Ha! You’re right, I should have known that. Did I at least get everything right?