by 
The Curious Bag
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Tires have a standardized sizing system that allows for easy replacement of same-sized tires and comparison of different-sized tires. Decoding tire sizes will allow you to compare the sizes of any tires.
Identification
A typical automobile tire size could be listed as P235/60R16. The P indicates passenger tire, 235 is the tire body width in millimeters, 60 is the aspect ratio or sidewall height percentage to section width, R stands for radial and 16 is the wheel diameter in inches.
Comparison
A 215 tire is 215 mm wide and the 235 is 20 mm wider. There are 25.4 mm to the inch, so the 235 aspect tire is 0.79 inches (20 divided by 25.4) wider than a 215 tire.
Considerations
A 235 tire with the same aspect ratio will be taller than a 215 tire. Twenty mm times 0.6 (60 aspect ratio) times 2 divided by 25.4 equals 0.94 inches. A 60 series 235 tire is just under one inch taller than a 215 tire.
Types
Taller tires with higher aspect ratios will have a greater height change when you go from a 215 tire to a 235 tire. A 235/70 tire is 1.1 inches taller than a 215/70 tire, but a 235/40 tire is only 0.63 inches taller than a 215/40 tire.
Fun Fact
To get a wider tire and keep the same tire height and wheel diameter, use a lower aspect ratio. A 235/55 tire is the same height as a 215/60 tire but is 20 mm or 0.79 inches wider.
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