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Tire size compatibility calculator

Check whether an alternative tire size is safe to fit your vehicle. Computes overall diameter delta, speedometer error at 30/45/60/75 mph, and load/speed rating verdict against the ETRTO ±3% safe-fit rule.

By Mark Bishop · Last verified 2026-05-17

How this calculator works

Every tire size encodes its overall diameter in three numbers. The first is the section width in millimetres (the tread footprint, measured wall to wall). The second is the aspect ratio (sidewall height as a percentage of section width). The third, after the R, is the rim diameter in inches. From these three numbers the calculator derives overall diameter using the ETRTO 2024 Standards Manual §2.3 formula:

overall_diameter_mm = rim_inches × 25.4 + 2 × (width_mm × aspect_percent / 100)

For your default example of 225/65R17, that resolves to 17 × 25.4 + 2 × (225 × 0.65) = 431.8 + 292.5 = 724.3 mm. The proposed 235/60R17 resolves to 431.8 + 2 × 141 = 713.8 mm. The delta is (713.8 − 724.3) / 724.3 = −1.45%. Inside the ±3% safe-fit window, this alternative is considered safe.

The speedometer reading at 60 mph indicated becomes 60 × (713.8 / 724.3) = 59.13 mph true. Your speedometer would over-report your true ground speed by 0.87 mph, well within the SAE J1349 ±4% tolerance for factory speedometer accuracy.

Why ±3% is the universal threshold

The 3% threshold is not arbitrary. It is the largest overall-diameter change that keeps four downstream calibrations intact simultaneously: speedometer accuracy under SAE J1349 (which mandates ±4% factory tolerance, leaving ~1% headroom for tire wear), TPMS rev/mile tracking under NHTSA FMVSS 138, ABS rotational comparison under FMVSS 135, and adaptive-transmission shift logic on torque-converter automatics that infer road speed from input-shaft rpm and wheel circumference.

Inside ±3%, the four systems remain within their factory programming. Outside ±3%, you start to see a stack of symptoms: speedometer reading drifts outside the J1349 envelope; TPMS may throw "calibration required" warnings even after a standard relearn; AWD viscous couplings start absorbing heat from front-rear rev mismatches; transmission upshift points drift.

Worked example: a typical Civic owner asks about 215/55R17 → 225/45R18

A common upgrade for a 10th-gen Honda Civic Sport on 17" alloys is the 18" wheel package from the higher-trim Civic Touring. The Touring OEM tire size is 225/45R18. We check geometric compatibility:

This is exactly the kind of plus-size step OEMs design wheel packages around — the diameter is intentionally preserved so the same speedometer calibration covers both options on the assembly line. The size pair appears in Honda's official wheel-fitment documentation as an interchangeable set.

What this calculator does not check

Geometry is necessary but not sufficient. The calculator does not verify wheel offset (the distance from the wheel's mounting face to its centerline), bolt pattern, hub bore, or fender clearance. These are mechanical fitment constraints that depend on your specific chassis. For a complete fitment check, cross-reference the size explanation page, your vehicle's OEM fitment page, and the manufacturer's owner-manual placard.

Sources & methodology

Last verified 2026-05-17 against the standards below.

  1. ETRTO 2024 Standards Manual §2.3 (section width, aspect ratio, overall diameter formula). European Tyre and Rim Technical Organisation, Brussels.
  2. Tire & Rim Association 2025 Yearbook, Table 1-2 (load index → maximum load equivalence). T&RA, Copley OH.
  3. ISO 4000-1:2021 Passenger car tyres and rims — Part 1: Tyres. International Organization for Standardization.
  4. SAE J1349 (Engine Power Test Code / speedometer accuracy reference). SAE International.
  5. NHTSA FMVSS 138 Tire Pressure Monitoring Systems — rev/mile calibration requirements. https://www.nhtsa.gov/laws-regulations/fmvss
  6. Tire Industry Association Technical Bulletin TB-2019-04, plus-sizing guidance.
  7. Honda Civic 10th-generation factory owner's manual & wheel-package option documentation.

FAQ

What is the safe tolerance when switching tire sizes?
The universally cited safe-fit tolerance is ±3% overall diameter, sourced from ETRTO 2024 Standards Manual §2.3 and adopted by every OEM tire-fitment guide we have reviewed. Inside ±3%, your ABS, speedometer, traction control, and AWD viscous coupling remain within factory tolerance bands. Above ±3%, expect speedometer errors above 4% (the SAE J1349 limit), TPMS calibration warnings, and potentially incorrect transmission shift points on adaptive transmissions.
Why does load index matter when changing tire size?
The load index is the maximum weight (per Tire & Rim Association 2025 Table 1-2) that a single tire can support at the rated inflation pressure. Fitting a tire with a lower load index than the OEM specification reduces total axle capacity. For passenger cars this rarely matters at typical loads; for trucks, vans, and any vehicle that tows or hauls, dropping load index can void the manufacturer's safety margin. The calculator marks an alternative as Caution when the load index is below your base size, even if the diameter is fine.
When does the ±3% rule break down?
Three cases push you toward the conservative ±2% strict toggle. First, electric vehicles with regenerative braking — the regen torque map is calibrated to OEM wheel circumference and drifts measurably outside ±2%. Second, AWD vehicles with a viscous center differential — even small revs-per-mile differences front-to-rear add heat to the coupling. Third, vehicles with active TPMS that re-learns rev/mile on every drive cycle — these throw warning lights at smaller deltas than the federal FMVSS 138 threshold.
Does this work for plus-sizing to a larger wheel?
Yes. Plus-sizing is just a special case of the same calculation: you increase rim diameter, drop the aspect ratio, and typically add 10mm of section width per +1 rim step to keep overall diameter within ±3%. The dedicated plus-size calculator handles this with one-click +0/+1/+2/+3 buttons; for arbitrary size changes, use this compatibility tool.
Should I trust this over my dealer's recommendation?
Use both. This calculator validates the geometry — it tells you whether a candidate size is dimensionally safe. Your dealer or manufacturer's tire-fitment guide may impose additional constraints (run-flat requirement, EV-specific marking, wheel-package option codes) that are not visible from the size alone. Always cross-check with the vehicle owner's manual placard before purchase.