How this calculation works
Width and aspect ratio together set the nominal sidewall height. A larger rim does not automatically make the complete tire larger, because a lower aspect ratio can offset it. This is why comparing rim inches alone misses the actual geometric change.
Diameter = rim inches × 25.4 + 2 × width × aspect ratio / 100; speed ratio = new diameter / original diameterA worked example
205/55 R16 gives 631.9 mm nominal diameter; 225/45 R17 gives 634.3 mm. At 100 km/h indicated the geometric speed is about 100.38 km/h.
Good to know
A larger circumference travels farther on each revolution. If the vehicle counts revolutions using its original calibration, the geometric road speed rises relative to the indication. Actual tire dimensions and rolling behaviour vary; this comparison is not permission to fit an alternative size.
Reference: Michelin — tire sizing and compatibility (opens in a new tab) ↗
Read the sidewall numbers as a connected system
Width and aspect ratio together set the nominal sidewall height.
A larger rim does not automatically make the complete tire larger, because a lower aspect ratio can offset it. This is why comparing rim inches alone misses the actual geometric change.
Tire circumference also enters the Gear ratio engine RPM and road speed calculator. Use the new nominal diameter there to compare the ideal relationship between engine rotations and road speed.
Keep instrument direction clear
A larger circumference travels farther on each revolution.
If the vehicle counts revolutions using its original calibration, the geometric road speed rises relative to the indication. Actual tire dimensions and rolling behaviour vary; this comparison is not permission to fit an alternative size.
Compare an identical tire first
Set the proposed size equal to the original size. Diameter change, axle-height change and geometric speed difference should all become zero. This baseline is useful before comparing a real alternative: it checks that each aspect ratio belongs to the intended width and that inches were not entered as millimetres.
Then change only the new rim diameter. Each added rim inch contributes 25.4 mm to overall diameter before sidewall changes. The axle moves by half the diameter difference, because its height is the radius. A negative result means the axle would sit lower in the nominal geometric comparison.
Work through the entered example
205/55 R16 gives 631.9 mm nominal diameter; 225/45 R17 gives 634.3 mm. At 100 km/h indicated the geometric speed is about 100.38 km/h.
Start with this case, then change one input and calculate again. Keep the other assumptions fixed so the result has an interpretable cause. The result table exposes intermediate quantities and units rather than hiding them behind a single headline number.
Use the output in a real plan
A larger circumference travels farther on each revolution. If the vehicle counts revolutions using its original calibration, the geometric road speed rises relative to the indication. Actual tire dimensions and rolling behaviour vary; this comparison is not permission to fit an alternative size.
Before purchasing material or comparing alternatives, check the measurements at the point where they enter the calculation. A mathematically precise answer can still be a poor plan when the measured size, loss allowance or price basis is wrong. Save the input assumptions with any result you intend to reuse.
A few questions, answered
What is the calculation method?
Diameter = rim inches × 25.4 + 2 × width × aspect ratio / 100; speed ratio = new diameter / original diameter
Does this use live prices or automatic measurements?
No. All measurements, prices and allowances come from your entries. The example values are illustrative and are not a recommendation for your situation.
Can I reuse the result for a different case?
Change the inputs and calculate again. Read the stated units and limits; the assumptions must still apply to the new case.