How to Compare Two Tire Sizes by Overall Diameter
Define the decision you need to make
Comparing two tire sizes by overall diameter answers one specific question:
How much will the tire’s nominal outside diameter change from one size to the other?
That comparison helps evaluate speedometer effect, gearing, ride height, wheel-well clearance, and whether a proposed size remains close to the vehicle’s original tire geometry. It does not confirm that the tire will fit the wheel, clear the suspension, or work safely on a particular vehicle.
A complete fitment decision should therefore separate:
- Tire geometry — overall diameter, width, sidewall height, and circumference.
- Wheel and vehicle compatibility — wheel width, PCD, center bore, offset, brake clearance, and load requirements.
- Physical clearance — actual space inside the OEM envelope, including movement under steering, suspension compression, and loading.
A tire calculator can perform the arithmetic consistently, but the output is only as reliable as the specifications entered.
Gather and normalize the necessary specifications
Tire sizes normally appear in a format such as 225/45R17:
- 225 = nominal section width in millimetres
- 45 = aspect ratio, or sidewall height as a percentage of width
- R = radial construction
- 17 = wheel diameter in inches
Record both sizes in the same format before comparing them. Avoid comparing a metric tire size with a measurement taken from a catalog drawing or an unverified physical tire without noting the difference.
For each tire, collect:
| Specification | Why it matters |
|---|---|
| Section width | Determines sidewall calculation and lateral clearance |
| Aspect ratio | Determines sidewall height |
| Wheel diameter | Determines the rim component of overall diameter |
| Load index | Confirms whether the tire can support the required load |
| Speed rating | Confirms suitability for the intended use |
| Approved rim-width range | Determines whether the tire can be mounted correctly |
| Manufacturer and model | Actual dimensions may vary between designs |
The size printed on the sidewall is nominal. Two tires with the same marked size can have different measured widths, tread widths, or overall diameters because of construction, tread design, inflation pressure, and approved wheel width.
Apply the overall-diameter formula
Convert the size into millimetres using these formulas:
Sidewall height
\[ \text{Sidewall height} = \text{section width} \times \frac{\text{aspect ratio}}{100} \]
Wheel diameter in millimetres
\[ \text{Wheel diameter} = \text{rim diameter in inches} \times 25.4 \]
Nominal overall diameter
\[ \text{Overall diameter} = (2 \times \text{sidewall height}) + \text{wheel diameter} \]
The calculator should also show the difference between the two tires:
\[ \text{Diameter difference} = \text{new diameter} - \text{reference diameter} \]
\[ \text{Percentage difference} = \frac{\text{new diameter} - \text{reference diameter}} {\text{reference diameter}} \times 100 \]
A positive result means the proposed tire is nominally taller. A negative result means it is nominally shorter.
Worked example
Compare 225/45R17 with 245/40R17.
| Measurement | 225/45R17 | 245/40R17 |
|---|---|---|
| Section width | 225 mm | 245 mm |
| Sidewall height | 101.25 mm | 98 mm |
| Wheel diameter | 431.8 mm | 431.8 mm |
| Nominal overall diameter | 634.3 mm | 627.8 mm |
The second tire is:
- 6.5 mm smaller in diameter
- Approximately 1.0% smaller
- Approximately 3.25 mm lower in nominal radius
- 20 mm wider nominally, although the actual section width depends on the tire model and measuring rim
The radius change is more relevant to static ride height and vertical clearance than the full diameter difference. A 6.5 mm diameter reduction does not create 6.5 mm of extra clearance at every location; the center of the tire moves by approximately half that amount.
Apply hard-blocker checks first
Do not treat a favorable diameter result as a fitment approval. Before comparing fine geometry, check conditions that can make the combination unusable regardless of diameter.
Confirm the wheel and tire interface
The proposed tire must be approved for the wheel’s width. A tire calculator cannot determine whether a tire is too narrow or too wide for a particular rim. Use the tire manufacturer’s approved rim-width range and account for the actual wheel specification.
If the tire is being installed on a different wheel, verify:
- Wheel diameter
- Wheel width
- PCD, also called bolt pattern
- Center bore
- Offset
- Fastener type and seat
- Brake and caliper clearance
- Load capacity of the wheel and tire
PCD and center bore are wheel-to-vehicle interface dimensions. They do not change the tire’s calculated diameter, but an incorrect PCD or unsuitable center bore can prevent the wheel from being mounted safely. Offset also changes the tire and wheel’s position relative to suspension components and the fender.
Confirm load and operating requirements
The replacement tire should meet or exceed the vehicle’s required load capacity and satisfy applicable speed-rating requirements. Check the vehicle placard, owner documentation, and manufacturer data rather than inferring requirements from diameter alone.
Also verify that the replacement tire’s construction and service category are appropriate. A numerically similar size is not automatically an acceptable substitute if its load index or approved application differs.
Compare geometry and supporting evidence
Once the hard blockers pass, compare the two tire sizes in a calculator using more than overall diameter.
Width
Nominal section width is useful for estimating lateral clearance, but it is not a guaranteed measurement. The actual tire may be wider or narrower than the marked size, and tread width may differ substantially from section width.
Check possible contact at:
- Inner sidewall and suspension components
- Strut, spring perch, or control arm
- Outer sidewall and fender lip
- Tread and wheel-arch liner
- Mud flaps and body trim
A wider tire can contact even when its overall diameter is smaller.
Sidewall height
Sidewall height affects more than diameter. A shorter sidewall usually reduces sidewall flex but may transmit more impact harshness and provide less rim protection. A taller sidewall increases the tire’s nominal radius and may reduce available clearance under compression.
Circumference and revolutions
Theoretical circumference is:
\[ \text{Circumference} = \pi \times \text{overall diameter} \]
A smaller tire travels a shorter distance per revolution. At a constant indicated speed, it may rotate faster than the reference tire. For a basic estimate, the speedometer or gearing effect follows the diameter ratio:
\[ \text{Actual speed} \approx \text{Indicated speed} \times \frac{\text{new diameter}}{\text{reference diameter}} \]
This is an estimate based on nominal dimensions. Actual behavior varies with tire pressure, load, wear, and loaded radius.
OEM envelope and system requirements
Compare the proposed tire against the vehicle’s original equipment envelope, not just one OEM size. The envelope may include approved ranges for diameter, width, wheel width, load index, speed rating, and wheel offset.
If the vehicle uses systems that compare wheel speeds—such as ABS, traction control, or all-wheel drive—maintaining closely matched rolling dimensions may be important. For vehicles with multiple driven axles, do not assume that a small nominal difference is acceptable without vehicle-specific guidance.
Identify missing data and uncertainty
A calculator result should state what is known and what remains uncertain.
Useful confidence levels include:
- Nominal comparison: Based only on the sidewall size.
- Catalog comparison: Includes the specific tire model’s published dimensions.
- Measured comparison: Uses actual mounted dimensions on the intended wheel, at a defined pressure and load.
- Physical verification: Confirms clearance on the actual vehicle through steering and suspension movement.
Published overall diameter may be measured under a particular standard and condition. It may not equal the loaded radius in service. Tire wear reduces diameter, while inflation pressure and load alter the tire’s shape. Wheel width can also affect measured section width.
If the proposed size is close to a clearance limit, do not rely on rounding. Obtain the exact tire model’s technical data and inspect the vehicle physically.
Turn the result into a purchase or test-fit action
Use the comparison to create a decision record:
- Reference tire size and exact model
- Proposed tire size and exact model
- Nominal overall diameter for each
- Diameter and percentage difference
- Nominal width difference
- Sidewall-height difference
- Load index and speed rating
- Approved rim-width range
- Wheel width, offset, PCD, and center bore
- Known clearance risks
- Data still requiring verification
If the geometry is comfortably within the vehicle’s documented OEM envelope and all wheel, load, and interface checks pass, the result can support a purchase decision. If the size is outside that envelope or clearance is uncertain, treat it as a test-fit project rather than a calculator-approved replacement.
For a physical verification, inspect at full steering lock in both directions and check clearance through suspension compression where safely possible. Look for contact marks, not just visual gaps. Account for tire growth, steering movement, body flex, passengers, cargo, and chains or other required equipment.
The calculator establishes the nominal geometry. Structured fitment data and physical verification determine whether that geometry works on the actual vehicle.