Compare current and proposed metric tire sizes for diameter, circumference, ride height, revolutions and speedometer effect.
Tire size comparison calculator
Decode two metric tire sizes from width, aspect ratio and rim diameter, then compare their nominal geometry.
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Calculated result
Enter your vehicle, trip or charging details and choose the calculate button.
Methodology
Sidewall height is section width × aspect ratio ÷ 100. Overall diameter is rim inches × 25.4 plus two sidewalls. Circumference is π × diameter. Speed and distance corrections use the ratio of proposed diameter to current diameter; ride-height change is half the diameter change.
Worked example
A 225/45R17 tire has a nominal diameter of about 634.3 mm. A 235/40R18 is about 645.2 mm, roughly 1.72% larger, so 60 indicated corresponds geometrically to about 61.0 in the same speed unit.
Reading a tire code
In 225/45R17, 225 is nominal section width in millimetres, 45 is sidewall height as a percentage of width, and 17 is rim diameter in inches.
Formula reference
Tire Size Comparison methodologyRelated automotive tools
Common questions
Sidewall height is 225 × 45%. Overall diameter adds two sidewalls to the 17-inch rim after converting the rim to millimetres.
A larger circumference travels farther per wheel revolution. Estimated actual speed equals indicated speed multiplied by new diameter divided by current diameter.
It is half the difference in overall diameter because only the radius below the axle changes nominal ground clearance.
No. Width, approved rim range, offset, brakes, suspension, fenders, load rating, speed rating and vehicle systems must also be checked.
Nominal geometry does not capture manufacturer design, approved measuring rim, inflation, load, tread wear or mounted deflection.
This calculator compares nominal P-metric geometry. It does not approve a tire, wheel or modification for a vehicle and cannot predict the exact mounted rolling radius. Confirm the vehicle placard, manufacturer specifications, load and speed ratings, rim width, clearance and applicable law with a qualified tire professional.