Enter the two tyre markings

How metric tyre size geometry is calculated

A marking such as 225/55 R17 gives three geometric inputs. The nominal section width is 225 millimetres. The sidewall height is 55% of that width. The wheel diameter is 17 inches, converted at 25.4 millimetres per inch.

These are nominal calculations. Installed dimensions vary by tyre model, approved rim width, inflation, load, wear and manufacturing tolerance. Use manufacturer technical data when millimetres matter.

Ride-height change is half the diameter difference because the axle sits one radius above the road. A 12 mm larger diameter raises nominal axle height by 6 mm, before deflection. It does not prove body clearance or legal suspension height.

Use the diameter screen as a comparison, not approval

The selected ±2% or ±3% band is a user-facing geometry screen, not a South African legal limit. It highlights a proposed overall-diameter change that deserves closer investigation. Passing the screen does not prove that the tyre, rim or modification is allowed.

Start with a manufacturer-approved tyre size for the exact vehicle, axle, wheel and model derivative. Confirm rim-width range, load and speed rating, clearance at full steering and suspension travel, braking and stability-system compatibility, and speedometer behaviour.

South African vehicle standards, roadworthiness rules and insurer or manufacturer conditions can depend on more than nominal diameter. Obtain written fitment guidance from a competent tyre professional or vehicle engineer where the replacement is outside approved documentation.

Speedometer and odometer effects

A larger circumference travels farther per wheel revolution. If calibration were unchanged, actual road speed would be higher than indicated by the proportional difference. A smaller tyre produces the opposite theoretical effect. The calculator shows the ratio at the entered indicated speed.

This is not a speedometer compliance test. Vehicle systems can obtain speed from wheel sensors and apply calibration or electronic compensation. Tyre growth, pressure, wear and instrument tolerance also matter. Test and calibrate using an approved method after fitment.

Odometer distance changes by the same circumference ratio in this simple model. If the proposed diameter is 2% larger, an odometer calibrated to the original may under-record actual distance by roughly 2%. That can affect service timing and trip calculations.

Never use the calculator to justify exceeding a speed limit. Drive to a verified compliant speed indication and allow for uncertainty.

Check the complete wheel and tyre fitment

Match tyres across an axle and follow vehicle instructions for all-wheel drive. Large circumference differences between tyres can affect driveline components and stability systems. Temporary spare instructions can be different.

Confirm wheel stud pattern, centre bore, fasteners, load rating and brake clearance. A tyre that fits a wheel does not prove the wheel fits the vehicle.

Size changes also change vehicle behaviour

A wider tyre may alter steering effort, tramlining, aquaplaning behaviour, rolling resistance and clearance. A lower-profile sidewall may sharpen response but reduce impact compliance and wheel protection. Results depend on tyre construction and compound, not size alone.

Increasing overall diameter changes effective gearing. Engine speed at a given road speed may fall, while acceleration and braking leverage can change. Larger, heavier wheel-and-tyre assemblies increase unsprung and rotational mass.

Fuel use and electric range can change with mass, aerodynamics, rolling resistance and pressure. The calculator does not estimate those effects. The new tyre’s label, testing and real use are more informative than width alone.

For towing, confirm tyre and wheel load capacity at actual axle loads and pressures. The towing capacity calculator cannot compensate for an underrated tyre.

A safer replacement process

Start with the placard and owner’s manual. Identify manufacturer-designated sizes, pressures and load requirements. Ask a qualified tyre professional to confirm the proposed product and rim are compatible with the vehicle and intended use.

Check state or territory modification guidance before purchase. If engineering approval is required, obtain it before committing. Notify the insurer of modifications as required and confirm cover in writing.

After installation, verify clearance, wheel fastener torque, pressure monitoring operation and speedometer accuracy. Recheck torque when instructed. Inspect for rubbing after normal steering and suspension movement.

Maintain pressure when tyres are cold using vehicle guidance adjusted for load where specified. Rotate, inspect and replace based on condition and age as well as tread depth. A mathematically compatible tyre can still be unsafe when damaged, aged or incorrectly inflated.

Read the markings the geometry fields leave out

A complete tyre description usually includes more than width, aspect ratio and rim diameter. A load index and speed symbol follow the size on many passenger tyres. Construction, reinforced or extra-load status, run-flat identification, original-equipment markings, manufacturing date and approval marks can also appear. This calculator does not interpret any of them.

The service description must be appropriate for the vehicle and its maximum loads. Do not assume a physically larger tyre has a higher load capacity, and do not replace an extra-load or specialist tyre with a superficially matching standard tyre without approval. The pressure required to carry a load depends on the product and vehicle application.

Directional tyres must rotate in the marked direction. Asymmetric tyres have an inside and outside orientation. Some vehicles use staggered front and rear sizes, so rotation options are limited. Mixing construction, pattern or wear can affect handling, particularly on wet roads and vehicles with sensitive all-wheel-drive systems.

The manufacturing date code helps identify age but does not provide a universal expiry date. Heat, storage, ultraviolet exposure, damage and use affect condition. Follow the tyre and vehicle manufacturer’s inspection and replacement guidance, and have cracking, bulges, vibration or repeated pressure loss examined promptly.

Work through a size comparison before shopping

First, calculate the manufacturer size and check that the nominal diameter matches the technical information reasonably closely. Second, calculate the proposed size and note diameter, sidewall and circumference differences. Third, use the reference screen only as a prompt for regulatory research.

Next, obtain the proposed tyre’s actual dimensional data and approved rim-width range from its manufacturer. Compare wheel offset, track, guard and suspension clearance. Confirm load capacity at the required pressure and the applicable speed category. A professional should inspect full steering lock and suspension movement, not only the parked gap.

Finally, compare vehicle behaviour and operating purpose. A replacement for quiet highway use may prioritise differently from a tyre for towing, gravel, snow or track activity. Size is one specification among compound, construction, tread, heat performance and wet braking. Select the complete product, not merely the arithmetic.