How this tire size calculator works
Enter a tire’s three numbers — section width in millimeters, aspect ratio as a percentage, and wheel diameter in inches — and the calculator returns the overall diameter, sidewall height, circumference, and revolutions per mile. You can also type a full size such as 225/45R17 into the top box to fill the fields automatically.
Reading a tire size
A sidewall code such as 225/45R17 packs three measurements into one string:
- 225 — section width in millimeters (the tire’s width from sidewall to sidewall).
- 45 — aspect ratio: the sidewall height is 45% of the width, so 0.45 × 225 = 101.25 mm.
- R — radial construction.
- 17 — wheel (rim) diameter in inches.
Overall diameter is the wheel diameter plus two sidewalls: 17 in + 2 × 101.25 mm works out to about 24.97 inches.
What overall diameter and revs per mile tell you
Overall diameter sets your ground clearance, how the speedometer reads, and how the gearing feels. Revolutions per mile is how many times the tire turns to cover one mile — it is the figure your speedometer and odometer actually count, so a change here is what throws those readings off.
When tire size matters
Check these numbers before buying replacement tires, plus-sizing to a larger wheel, or fitting a different size to make sure the new tire clears the bodywork and keeps the speedometer honest.
Comparing two tire sizes
To see how switching to a new size changes your speedometer reading and whether it stays within a safe range, use the Tire Size Comparison Calculator.
Frequently asked questions
How do I find my tire size?
It is printed on the tire sidewall as a code like 225/45R17, and also listed on the placard inside the driver’s door jamb.
What is revolutions per mile?
It is the number of full rotations a tire makes to travel one mile, calculated as 63,360 inches divided by the tire’s circumference in inches.
Does a bigger tire change my speedometer?
Yes. A taller tire covers more ground per turn, so the speedometer reads slower than your true speed; a shorter tire reads faster. The comparison calculator shows the exact percentage.