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Identify the requirement before comparing tires
All-wheel-drive and four-wheel-drive systems do not share one universal tire-mismatch allowance. Vehicle documentation may specify matching sizes, models, circumference or wear limits, and replacement procedures. The practical decision is whether the proposed tires satisfy the requirements for that exact vehicle and configuration.
Start with the owner’s manual and applicable manufacturer guidance. Record year, model, powertrain and any approved staggered arrangement. If the requirement is unclear, obtain vehicle-specific guidance. A calculator cannot fill that gap by declaring a generic 3% rule acceptable. A small number can still be outside a particular requirement, and a permitted factory arrangement may not be symmetric.
Separate nominal diameter from rolling behavior
A tire size code supplies nominal geometry. Overall diameter published for an exact tire is a product dimension under stated conditions. Rolling circumference describes distance traveled per revolution under operating conditions. These are related, but not interchangeable.
The Tire Size calculator compares code-derived nominal diameters. It is useful for screening a proposed change, not establishing an AWD tolerance. The Manufacturer Specifications guide explains which data fields to record and how measurement conditions affect the comparison.
Why wheel-speed relationships matter
Different rolling distances can create different wheel rotational speeds at the same road speed. A drivetrain or control system may encounter a persistent difference that is not simply a transient turn or wheel slip. How it responds depends on its design and calibration.
This principle does not identify the permitted difference or predict damage on a particular vehicle. Avoid applying a mechanism explanation as a universal numerical threshold. Manufacturer requirements take priority over an internet percentage because the calculator has no model of that drivetrain’s design, temperature behavior or control strategy.
Example 1: two nominal sizes
Compare 225/45R17 and 245/40R18. Their nominal diameters are 634.30 and 653.20 mm. The second is about 2.98% larger. At equal actual road speed, its geometric wheel-revolution rate is about 2.89% lower because revolution rate scales inversely with diameter.
diameter ratio = 653.20 ÷ 634.30 = 1.0298
relative revolution rate = 634.30 ÷ 653.20 ≈ 0.9711Those calculations are not an approval to mix these sizes across axles or replace one tire with another. They merely expose a geometric difference. A generic “within 3%” statement would conceal the unanswered vehicle-specific requirement and exact product behavior.
Example 2: identical size code, different wear
Consider an idealized 650 mm diameter tire. If tread loss reduces radius by 3 mm uniformly, ideal diameter decreases by 6 mm to 644 mm. The geometric diameter difference is approximately 0.92%. That shows why the same sidewall code does not establish an unchanged dimension after wear.
This is an illustrative geometric model, not a field measurement method or allowable wear threshold. Real tread variation and rolling behavior need appropriate assessment. Do not convert this example into a universal rule for replacing one tire, shaving a tire or mixing wear levels.
Match the data to the requirement
If vehicle guidance specifies tire size and model matching, record both. If it specifies tread-depth criteria, obtain measurements using the appropriate method. If it specifies circumference, determine how that figure should be measured or sourced. A published unloaded diameter is not automatically a substitute for a required rolling-circumference check.
Keep the requirement’s exact wording and source with the comparison. Record tire model, full service description, pressure conditions and relevant wear data. Do not assume a requirement for one generation or drivetrain transfers to another with the same badge. If a document has exceptions, preserve them rather than reducing it to one convenient number.
Approved staggered packages are a separate case
Some vehicles use different front and rear tire dimensions as an approved package. That does not mean arbitrary diameter combinations are acceptable. The manufacturer’s arrangement and requirements should be evaluated as a complete package.
Similarly, changing all four tires to one different size does not automatically resolve every issue. Load capacity, approved rim width, body clearance, calibration and other vehicle requirements remain. The Setup Planner can compare geometry but does not confirm a drivetrain-approved arrangement.
Common mistakes
- Treating a nominal percentage as a manufacturer tolerance.
- Assuming identical size codes establish identical rolling characteristics.
- Ignoring wear when selecting a single replacement.
- Transferring guidance between model years or powertrains.
- Treating a factory staggered arrangement as permission for any staggered sizes.
- Using tire pressure changes to disguise an unresolved matching problem.
Pressure should follow appropriate operating guidance, not serve as an improvised diameter-matching control. This guide supplies no pressure adjustment to make incompatible tires acceptable. A temporary spare also has vehicle-specific operating restrictions; follow those rather than applying the nominal comparison to unrestricted use.
What the calculator still cannot see
It cannot identify the drivetrain, determine acceptable mismatch, inspect wear, measure rolling circumference or predict component response. It cannot diagnose a vibration, warning light or driveline concern from tire codes. Even a zero nominal difference leaves product and vehicle requirements to check.
If symptoms are present after a tire change, document the arrangement and obtain appropriate diagnosis. Do not assume either that tires caused the symptom or that a close calculator result rules them out. The distinction between dimensional screening and diagnosis remains important here, just as it does in the planned diagnostic-reference section.
Replacement decision checklist
Identify the exact vehicle and applicable requirement. Record all installed tire models, sizes, service descriptions and relevant wear measurements. Identify the proposed replacement and compare equivalent data. Obtain guidance where a requirement or measurement is missing.
Before ordering, confirm whether replacing one, two or all four is appropriate under that vehicle’s instructions. Verify the complete package’s ratings, rims and operating requirements. Keep records of the replacement and any calibration or inspection performed. A complete decision record is more useful than an unexplained percentage screenshot.
Keep an unresolved requirement visible
A useful comparison sheet can have a row labeled “vehicle tire-matching requirement” with its source and status. If the relevant guidance has not been obtained, mark the row unresolved. Do not populate it with a tolerance from a different vehicle simply to finish the worksheet.
This practice also helps a tire specialist evaluate the request. Present the exact vehicle configuration, installed models, sizes and wear information rather than asking whether a generic percentage is safe. The missing evidence may be a manufacturer requirement, an appropriate measurement or an exact product specification. Identifying that gap is a useful result of the planning process. It is more defensible than issuing approval from a nominal number whose relationship to the drivetrain requirement has not been established.
Related guides and sources
Read Tire Size Explained, Manufacturer Specifications, and Wheel & Tire Buying Workflow. USTMA replacement guidance is a general starting point; the vehicle’s own documentation governs specific requirements. All numerical examples here are original illustrations, not approved combinations or tolerances.
Content and numerical examples checked October 9, 2026.