Compare nominal tire dimensions, visualize size changes, calculate speedometer and odometer differences, and review what to verify before buying.
Full Tire Size GuideCompare nominal tire dimensions and understand the effects of changing size.
Enter a size such as 225/45R17. P and LT prefixes are accepted.
Transparent methodology
The calculator parses each tire code, converts the wheel diameter to millimetres, calculates both sidewalls, and compares nominal diameter and circumference. It then uses the circumference ratio to estimate speedometer and odometer differences.
Separate width, aspect ratio, and wheel diameter.
Find sidewall, diameter, circumference, and revolutions.
Estimate clearance, speed, and distance changes.
Check the vehicle, wheel, ratings, and clearances.
Reading the sidewall
225 is the nominal section width in millimetres. 45 means the sidewall height is 45% of that width. R identifies radial construction, and 17 is the wheel diameter in inches.
The calculation converts the wheel diameter to millimetres and adds 2 sidewalls. For 225/45R17, each nominal sidewall is 101.25 mm and the nominal overall diameter is 634.3 mm.
Diameter is not fitment
A diameter difference near 3% is often used as a comparison reference, but it cannot verify physical or operational compatibility. A similar-diameter tire may still contact the suspension or fender, sit outside the approved rim-width range, or have an inadequate load or speed rating.
Before purchasing, confirm the tire manufacturer’s measured specifications, approved rim widths, vehicle requirements, wheel offset, brake and suspension clearance, full steering and suspension travel, TPMS requirements, and any AWD circumference restrictions.
Speed and distance
A larger replacement travels farther per revolution, so nominal actual speed and distance are higher than indicated. A smaller replacement does the opposite. Real rolling circumference can differ from nominal geometry because of load, pressure, tread, construction, and wear.
Tire-size questions
Sidewall height is the section width multiplied by the aspect ratio. Overall nominal diameter is the wheel diameter plus 2 sidewalls. Tire and rim measurements use different units, so the wheel diameter is converted from inches to millimetres first.
No. Diameter difference is only one geometry check. Rim width, wheel offset, suspension and fender clearance, load index, speed rating, TPMS, drivetrain requirements, and manufacturer guidance must also be verified.
A larger circumference travels farther per wheel revolution, while a smaller circumference travels less. The calculator compares nominal circumferences to estimate actual speed at a selected indicated speed.
No. Tire codes provide nominal dimensions. Measured dimensions and rolling circumference can vary by tire model, manufacturer, rim width, pressure, load, tread depth, and wear.
Yes. Open Advanced Options to compare up to 3 replacement candidates against the same original tire.
P and LT identify different tire service categories and construction requirements. The prefix is retained in the result, but it does not change the nominal geometry formula. Always verify the required load and speed ratings separately.
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