Plus-sizing wheels: tyre diameter and speedometer changes

Plus-sizing means increasing wheel diameter while choosing a tyre profile intended to keep overall rolling size close to the original. Moving from a 17 inch rim to an 18 inch rim adds 25.4 mm to the wheel diameter. To keep the same nominal outside diameter, the combined upper and lower sidewalls would need to become 25.4 mm shorter, or 12.7 mm shorter each.

Available tyre sizes rarely give an exact mathematical match. Width, aspect ratio, load rating and the tyre maker's range limit your choices. Start with the vehicle's approved alternative sizes if they exist. A size sold on a different trim is useful evidence to investigate, but differing brakes, axle loads or suspension can still matter.

Going wider on the same wheel diameter is sometimes called plus zero. That can also change overall tyre diameter if you retain the same aspect ratio. Read the entire tyre code and calculate the result rather than assuming that a shared wheel diameter keeps gearing and speed readings unchanged.

Take 225/45 R17 as a hypothetical starting size. Its nominal diameter is 17 × 25.4 + 2 × 225 × 0.45 = 634.3 mm. Compare 225/40 R18: 18 × 25.4 + 2 × 225 × 0.40 = 637.2 mm. The difference is 2.9 mm, or about 0.46%. The radius increases by half that, 1.45 mm.

Now compare 235/40 R18. Its nominal diameter is 645.2 mm, a 10.9 mm or 1.72% increase over the original. The wheel diameter and aspect ratio match the first candidate, but the extra width adds sidewall height. That is why a wider tyre with the same profile number can change more than its appearance.

A larger rolling circumference covers more ground per wheel revolution. If the speedometer was accurate with the original tyres and its calibration remains unchanged, actual speed becomes higher than indicated after fitting larger tyres. Multiply the indicated speed by new diameter divided by old diameter to estimate the effect of size alone.

For the 235/40 R18 example, 60 × 645.2 ÷ 634.3 = about 61.03 mph. At an indicated 30 mph, the corresponding estimate is 30.52 mph. These figures do not include the car's existing speedometer error. If the instrument already overreads, the actual result will be different from a calculation that assumes perfect baseline accuracy.

Do not use a tyre change as an improvised speedometer correction. Obtain any necessary calibration or vehicle approval through a competent specialist. The odometer and other systems using wheel-speed information may also be affected. Keep the original and proposed sizes in your records so the change can be assessed consistently.

A frequently repeated diameter tolerance is not universal permission to fit a tyre. It does not check load capacity, speed rating, tyre-to-rim approval, wheel strength, brake clearance or steering movement. It also does not describe the vehicle manufacturer's requirements for all-wheel-drive systems, ABS or stability control.

A combination within 1% can be unsuitable because the wheel hits a caliper or the tyre cannot support the required load. Conversely, a vehicle may have a manufacturer-approved alternative that needs specific settings or components. Evidence for that exact application is more useful than a general percentage taken from a forum.

Michelin's guidance on mixing tyres explains that differences in circumference can affect wheel-speed interpretation. For an all-wheel-drive car, check the vehicle maker's requirements for size, tyre model and wear matching. Do not replace those requirements with a home-made front-to-rear percentage limit.

A shorter sidewall leaves less distance between the road and the wheel rim, but the resulting ride depends on pressure, tyre construction, wheel mass and suspension as well. Avoid promises that every larger wheel improves handling. Compare the complete wheel-and-tyre assembly and the roads you actually use.

Before choosing the larger size, price four suitable tyres and check availability. A visually appealing wheel can lock you into an expensive or limited tyre range. If the car is used through winter, confirm whether the size has appropriate seasonal choices rather than discovering that later.

Brakes can set the minimum wheel diameter, while rough roads, running costs and comfort may argue against the maximum. A smaller wheel with a suitable tyre may fit your use better even when a larger option is available. Decide what improvement you want to feel or see before buying dimensions for their own sake.

• Record the original approved tyre size and full service description.
• Check the vehicle's approved alternatives and any calibration requirements.
• Compare nominal diameters, then the exact tyre maker's specifications.
• Confirm tyre load, speed and approved rim-width requirements.
• Check wheel width, ET, centre bore, fastening and brake clearance.
• Trial-fit and inspect steering and suspension travel before road use.

Save the calculator result as a comparison, not a certificate. Ask the fitter to record the final tyre product, wheel part number and approved pressure information on the job sheet. If you later change tyre brand within the same size, check actual dimensions again where clearance is tight.

Use a visual preview to decide whether the change in proportion is worth pursuing, then keep the mechanical decision separate. A photograph can show you that the larger wheel overwhelms the body shape before you spend anything, but it cannot tell you whether the tyre clears the liner when the car is loaded.

• Calculate diameter and speedometer changes