CLEAR INPUTS. HONEST ANSWERS.

Calculation methodology

The tire geometry, fuel conversion, wheel offset and ownership cost formulas behind every calculator.

Tire dimensions

Sidewall (mm) = width (mm) × aspect ratio / 100 Wheel diameter (mm) = rim diameter (in) × 25.4 Overall diameter D (mm) = wheel diameter + 2 × sidewall Circumference C (mm) = π × D Revolutions / km = 1,000,000 / C Revolutions / mile = 1,609,344 / C Diameter change (%) = (Dnew / Doriginal − 1) × 100 Ground clearance / ride-height change = (Dnew − Doriginal) / 2 Actual speed = indicated speed × Dnew / Doriginal

These are unloaded nominal dimensions derived from the metric size marking. Actual measured size and loaded rolling circumference vary. The speed estimate isolates the tire-size change and assumes correct original calibration. It is not an instrument calibration test.

The supported metric envelope is 125–445 mm width, 20–100 aspect ratio and 10–26 inch rim diameter. The range is an input-validation boundary, not a list of available or approved sizes.

Comfort interpretation and alternatives

Sidewall height within 2% is labelled similar; a 2–10% change is slightly higher or lower; more than 10% is higher or lower. These editorial bands describe geometry, not measured comfort. Construction, pressure, compound, suspension, vehicle and wheel mass, and road surface all affect ride quality.

The alternatives search examines widths within ±30 mm in 10 mm steps, aspect ratios from 25 to 85 in 5-point steps, and wheels within ±2 inches of the original. Results must fit the selected 1%, 2% or 3% absolute diameter limit. These combinations are theoretical. Availability and vehicle compatibility must be verified separately.

Wheel offset

Inner position = wheel width (in) × 25.4 / 2 + ET (mm) Outer position = wheel width (in) × 25.4 / 2 − ET (mm) Inner clearance change = original inner position − new inner position Outer protrusion change = new outer position − original outer position

Positive inner-clearance change means more space to the suspension. Positive outer change means farther outward. Bead-seat geometry excludes flange thickness, tire bulge, spoke shape and brake clearance.

Fuel and international units

1 inch = 25.4 mm 1 mile = 1.609344 km 1 US liquid gallon = 3.785411784 litres 1 imperial gallon = 4.54609 litres L/100km = 100 / km/L US MPG = km/L × 3.785411784 / 1.609344 UK MPG = km/L × 4.54609 / 1.609344 Litres consumed = distance (km) / economy (km/L) Fuel cost = litres × price per litre

Daily, weekly and monthly distances are annualized using 365, 52 and 12 periods respectively. Results for those periods are averages based on the same convention. Currency selectors label amounts without currency conversion. Optional vehicle-data conversion helpers use mechanical hp (1 kW ≈ 1.34102209 hp) and torque (1 Nm ≈ 0.737562149 lb-ft); metric PS is a different power unit.

Hybrid and EV ownership

Blended electricity price = home tariff × (1 − public share) + public tariff × public share EV energy cost / km = battery kWh/100km / 100 / charging efficiency × blended price Annual operating cost = annual energy + maintenance + insurance Initial premium = alternative purchase + EV charger installation − petrol purchase Simple break-even years = initial premium / annual operating saving Selected-period net cost = initial cost + annual operating cost × years − resale

The crossover calculation handles zero savings, no payback and reversal of an initial price advantage. Its chart excludes resale because only a terminal resale estimate is entered. Selected-period net totals include resale. Values are undiscounted and assume constant prices and distance. Financing, taxes, incentives, fixed charging subscriptions, parking, tolls, unexpected repairs and battery replacement are not modelled. The default 90% charging efficiency is an editable assumption. Use 100% when the consumption input already includes charging losses.

Source references

All figures are rounded only for display. See the calculators for worked examples and limitations.