How to Use This Tool
Decode two metric tire sizes into diameter and indicated-to-actual speed difference. Compare old and new tire diameter, circumference and actual speed from width, aspect ratio, rim size and indicated speed.
The decision this tool supports
A tire can fit the rim yet change rolling circumference enough to bias speed, distance, gearing and clearance. This page keeps the decision bounded to circumference difference and the supporting outputs shown beside it. Tire Speed Difference does not import an account, infer a market rate, or silently substitute an industry average.
Inputs and units
The Tire Speed Difference calculation uses Old tire width, Old aspect ratio, Old rim diameter, New tire width, New aspect ratio, New rim diameter, Indicated speed. Keep all money values in one currency and all time, distance, mass, energy or volume entries in the unit printed beside the field. Mixing Tire Speed Difference scopes can produce a plausible number with the wrong meaning.
- Old tire width is entered in mm.
- Old aspect ratio is entered in %.
- Old rim diameter is entered in inches.
- New tire width is entered in mm.
- New aspect ratio is entered in %.
- New rim diameter is entered in inches.
- Indicated speed is entered in mph or km/h.
Formula and worked check
Overall diameter mm = 2 × width × aspect ratio + rim inches × 25.4. Circumference and actual speed change in the same diameter ratio. 225/65R17 to 235/60R18 changes nominal diameter from 724.30 to 739.20 mm, +2.1%; an indicated 60 corresponds to about 61.23 in the same speed unit. The Tire Speed Difference default is an executable known-answer case, not a benchmark or recommendation. Change one input and verify that the direction of circumference difference still matches the stated relationship.
How to interpret the result
Use the ratio as a geometry check, then confirm manufacturer-approved sizes, load index, speed rating and vehicle calibration. The additional Tire Speed Difference outputs expose the denominator, comparison, capacity or reverse value needed to audit the primary result instead of presenting one unexplained number.
Assumptions
- Both sizes use metric width/aspect notation and inch rim diameter.
- Nominal unloaded geometry approximates circumference ratio.
- Indicated and actual speed use the same unit.
Save the Tire Speed Difference input values and date with any material decision. A later Tire Speed Difference rerun is reproducible only when the same assumptions and units are available.
Limitations and safety boundary
Actual loaded rolling radius differs with pressure, load, tread and construction; the tool cannot certify fitment or legal speedometer tolerance. Tire Speed Difference is an estimate and cannot replace a contract, local code, licensed professional, calibrated measurement, lender statement or platform report where one governs the decision.
Source and privacy
The Tire Speed Difference definition or rule was checked against NHTSA — Tires on 2026-08-26. Recheck NHTSA — Tires when a specification or policy behind Tire Speed Difference can change. Tire Speed Difference arithmetic runs in this browser tab; ecech does not receive the values through a calculation API.
Sources & assumptions
Tool Spec v2 · verified 2026-08-26. Platform rules and fees can change; the editable inputs remain authoritative for your account.
Official references
- NHTSA — Tires (checked 2026-08-26)
Model assumptions
- Both sizes use metric width/aspect notation and inch rim diameter.
- Nominal unloaded geometry approximates circumference ratio.
- Indicated and actual speed use the same unit.
- Actual loaded rolling radius differs with pressure, load, tread and construction; the tool cannot certify fitment or legal speedometer tolerance.
