Tire Diameter Calculator
Turn a tire size like 225/45R17, LT265/70R17 or 33x12.50R15 into overall diameter, width, sidewall, circumference and revolutions per mile or km — then compare a second size for diameter difference, speedometer error and similar-height sizes on other rims.
How it works
Metric size W/A R D: sidewall (mm) = W × A ÷ 100 · diameter (in) = D + 2 × sidewall ÷ 25.4 · Inch size H x W R D: diameter = H, sidewall = (H − D) ÷ 2 · Circumference = π × diameter · Revs/mile = 63,360 ÷ circumference (in) · Revs/km = 1,000,000 ÷ circumference (mm) · Diameter change % = (new − old) ÷ old × 100 · Actual speed = indicated × new diameter ÷ old diameter
A metric size like 225/45R17 packs three numbers: the section width in millimetres, the aspect ratio (sidewall height as a percentage of that width) and the rim diameter in inches. The sidewall is 225 × 45% = 101.25 mm; add it above and below the rim and you have the overall diameter. Light-truck and off-road tires are often sold in inch (flotation) sizes such as 33x12.50R15, where the first number already is the diameter and the second the width, so the sidewall is half of the diameter minus the rim. You can type the code straight from the sidewall — prefixes like P or LT, speed letters like ZR and load/speed suffixes are understood — and an old code without an aspect ratio (185R14) is read as 82%. From the diameter come the circumference and revolutions per mile or km, which set your speedometer reading and effective gearing. Turn on the comparison to see how a new size changes diameter, ground clearance and speedometer accuracy: a taller tire covers more ground per turn, so the speedometer reads slow. The table of sizes within 3% shows plus-size options on bigger or smaller rims that keep the rolling height close to the original. Section width is the widest point of the inflated tire on its measuring rim, not the tread width.
Worked example
For 225/45R17: sidewall = 225 × 0.45 = 101.25 mm = 3.99 in. Diameter = 17 + 2 × 3.99 = 24.97 in (634 mm). Circumference = π × 24.97 = 78.44 in, so revolutions per mile = 63,360 ÷ 78.44 ≈ 808 (502 per km). Swapping to 245/40R18 gives 25.72 in, 0.75 in or 3.0% taller: at an indicated 60 mph you would really be doing about 61.8 mph, and the car sits 0.37 in higher.
Frequently asked questions
What does 225/45R17 actually mean?
225 is the section width in millimetres, 45 is the aspect ratio (the sidewall is 45% of 225 mm = 101 mm tall), R means radial construction, and 17 is the wheel diameter in inches. A P prefix marks a passenger tire, LT a light-truck tire; letters after the R or the rim size (like 94W) are the load index and speed rating, which do not change the size.
How much bigger can I go without problems?
Most makers suggest staying within about 3% of the original overall diameter. Beyond that the speedometer, odometer, ABS and transmission shift points drift, and a taller or wider tire can rub the fender liner or suspension at full lock. The comparison shows the percentage and the table lists sizes inside the 3% window.
How do I convert 33x12.50R15 to a metric size?
Pick the inch format or type the code. The calculator works out the sidewall, (33 − 15) ÷ 2 = 9 in, and the nearest metric size — about 315/75R15. Metric and inch sizes rarely match exactly, so compare the diameters before buying.
Why does a bigger tire make my speedometer read slow?
The speedometer counts wheel revolutions calibrated for the original tire. A taller tire covers more distance per turn, so you travel faster than the needle shows: 3% taller means about 61.8 mph at an indicated 60.
What are revolutions per mile used for?
It is how many times the tire turns per mile (or km), which feeds speedometer calibration, gear-ratio and regearing calculations and odometer accuracy. Tire makers publish a measured figure that is slightly higher than the geometric one because a loaded tire squashes a little.
Related calculators
Educational estimate — real figures vary by vehicle, load, conditions and manufacturer specs. Always follow your owner’s manual.