Tire Size Calculator
Turn a tire size like 225/45R17 into overall diameter, sidewall height, and speedometer error versus another size.
Overall diameter
24.97 in
Sidewall height is 101.3 mm (3.99 in), so the tire stands 24.97 inches tall.
Sidewall height
101.3 mm
Circumference
78.45 in
Revs per mile
808
How it works
A tire code like 225/45R17 packs three numbers: 225 is the section width in millimeters, 45 is the aspect ratio (the sidewall is 45% of that width), and 17 is the wheel diameter in inches. From those you can rebuild the tire's real size.
The calculator finds the sidewall height first — width times aspect ratio — converts it to inches, then adds two sidewalls to the rim to get overall diameter. Multiply by pi and you have the circumference, which also tells you how many times the tire turns per mile.
Turn on the comparison to check speedometer error before you change sizes. Your speedo is calibrated to the original tire's diameter, so a taller tire makes it read low (you're actually going faster) and a shorter tire makes it read high. Keeping the error under a couple of percent keeps things honest.
Frequently asked questions
What does 225/45R17 actually mean?
225 is the tire's width in millimeters, 45 is the sidewall height as a percentage of that width, the R means radial construction, and 17 is the wheel rim diameter in inches.
Why does a bigger tire throw off my speedometer?
Your speedometer counts wheel rotations and assumes the stock tire's circumference. A taller tire covers more ground per turn, so the gauge undercounts and reads slower than you're really going.
How much size change is safe?
Enthusiasts usually keep overall diameter within about 3% of stock. That limits speedometer error, keeps the wheel clear of the fender and suspension, and doesn't confuse the ABS and traction systems.