Free tool
What does 205/55 R16 actually measure?
The three numbers on the sidewall describe width, aspect ratio and rim diameter. Everything else is arithmetic.
Try the calculator
Tools
Reading the sidewall
The first number is the section width in millimetres. The second is the sidewall height as a percentage of that width. The third is the rim diameter in inches. Overall diameter is the rim plus two sidewalls, which is why a wider tyre with the same aspect ratio is also taller.
Circumference follows from diameter, and revolutions per mile follows from circumference. Those two numbers are what matter when comparing a replacement size.
Why a 3% difference matters
Changing overall diameter changes the speedometer, the odometer and effective gearing. A small difference is usually tolerated; a large one can affect clearance, ABS calibration and fuel consumption. Compare the new diameter with the original before buying.
Frequently asked questions
Does rim width matter?
Yes for fitment, but it is not part of the size code. The wheel must be within the tyre manufacturer's approved rim-width range.
Can I change the aspect ratio?
Yes, but keep the overall diameter close to the original. A lower aspect ratio on a wider tyre can preserve the diameter while changing ride and gearing.
What is the speedometer error?
It is roughly the percentage change in diameter. A larger tyre makes the speedometer read lower than the true speed.
Why does revs per mile matter?
It is used by gearing, traction-control and odometer calculations, and it is the clearest single number for comparing two candidate sizes.