Before changing tire or wheel size

Start with the vehicle, not the current sidewall alone

Check the vehicle placard, owner’s manual and applicable manufacturer guidance. A used vehicle may already have a different size fitted. Record the approved size for each axle, including any documented front and rear differences.

Michelin recommends checking vehicle-manufacturer restrictions before selecting an alternate size. Its plus-sizing explanation describes changing rim diameter while keeping overall tire diameter close; it does not supply a universal percentage that approves a replacement.

Keep the complete tire and wheel specification

A size such as 225/45R18 contains width, aspect ratio and rim diameter, but a complete tire marking has more information. Check the required load and speed ratings and the tire manufacturer’s approved rim-width range. Preserve relevant construction and service markings instead of assuming matching geometry means equivalent tires.

For a wheel change, verify the rim diameter, bolt pattern, hub bore, fastening system and vehicle approval. Nominal width and ET locate a simplified wheel relative to its mounting face; they do not describe spoke shape, rim lips or tire bulge.

• Vehicle placard and manufacturer-approved alternatives for each axle
• Complete tire marking, product specification and approved rim widths
• Wheel and fastening specifications, brake and suspension clearance
• Vehicle-specific AWD, staggered-fitment and recalibration requirements

Use TireSpan to ask specific questions

The tire comparison gives nominal diameter, width, sidewall and circumference changes. The offset calculator shows inward and outward movement from a fixed mounting plane. Bring the original inputs and a downloaded report when discussing a proposed change with a qualified tire professional.

Ask how clearance is checked at full steering lock and throughout suspension travel, and which product dimensions and vehicle limits apply. A static drawing or a positive inner-space figure cannot answer those installation questions.

A percentage is a measurement, not permission

A small diameter difference does not verify load capacity, braking-system behavior, body clearance or manufacturer approval. A larger difference is not a reason to invent a different universal threshold. Keep the calculated quantity and the independent approval evidence side by side.

TireSpan has no vehicle fitment database and does not recommend tire pressures or certify replacement sizes. Use the applicable manufacturer information and professional inspection for the installation decision.

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