Why do we measure the sidewall height as a percentage of tire width? For example, a 215/65-16 and a 235/60-16 tire have the same sidewall height, but the numbers are different.
Does this strange numbering system serve any purpose?
The 235/60-16 tire has a wider tread than the 215/65-16, 235 mmm to 215 mm. The 60 series tire may also require a wider wheel and my be too wide for certain applications that the 65 series tire will work just fine. More info on tire sizing can be found here.
No, but why is the sidewall height expressed as a percentage of the tire’s width, as opposed to just writing the measurement directly?
For example, 235/140-16 instead of 235/60/-16.
I’ve wondered about this myself.
My guess is that some basic traits of the tire’s handling are related to the aspect ratio. A high AR would flex more on curves but probably cushion more on bumps. A low AR is desired for high-speed cornering, since the tire would have less lateral distortion. I assume this is proportional across various tire sizes (e.g., a 195/60 handles pretty much like a 235/60, but differently from a 195/40 or a 195/75). When the designation system was designed, this was probably considered more helpful information than the actual sidewall height.
Standardization. From the same site I linked above, there is more info here.
Thanks for the answers!
Another related question: Why millimeters for tire width, but inches for rim diameter? Why not use the same system for both?
(I know that for some special tires width is measured in inches, but that’s an exception)