Tire Size Calculator
Size Difference Check
+1.7%
Within the common ±3% diameter guideline
New tire diameter is 1.7% larger.
Diameter similarity does not guarantee vehicle fitment or suitability.
- Section width change
- +10.0 mm
- Sidewall change
- −7.3 mm
- Overall diameter change
- +10.9 mm
- Theoretical radius change
- +5.5 mm
60 mph indicated ≈ 61.0 mph modeled actual.
Nominal geometry, not measured ground clearance. Guideline classification uses the unrounded difference; a displayed 3.0% can be just outside it.
See the difference
Both tires use the same scale. Circles show tire and rim diameter; front profiles show nominal width. These are unloaded dimension diagrams, not tread or contact-patch models.
What changes?
- 10.0 mm wider
- 7.3 mm shorter sidewall
- 10.9 mm larger overall diameter
- 5.5 mm increase in theoretical radius
- New tire diameter is 1.7% larger.
The radius difference is the theoretical static ride-height contribution from tire geometry alone. Actual loaded radius and vehicle ground clearance can differ.
When your speedometer says…
A larger circumference travels farther per revolution. These modeled speeds assume calibration to the current tire size; they do not assume every factory speedometer is perfectly calibrated.
| Indicated | Modeled actual |
|---|---|
| 30 mph | 30.5 mph |
| 50 mph | 50.9 mph |
| 60 mph | 61.0 mph |
| 70 mph | 71.2 mph |
Detailed measurements
Dimensions in millimeters. Change display units above; the tire codes always use mm / % / inches.
| Measurement | Current | New | Difference |
|---|---|---|---|
| Tire size | 225/45R17 | 235/40R18 | — |
| Section width | 225.0 mm | 235.0 mm | +10.0 mm |
| Sidewall height | 101.3 mm | 94.0 mm | −7.3 mm |
| Rim diameter | 431.8 mm | 457.2 mm | +25.4 mm |
| Overall diameter | 634.3 mm | 645.2 mm | +10.9 mm |
| Circumference | 1,992.7 mm | 2,027.0 mm | +34.2 mm |
| Revolutions / mile | 808 | 794 | — |
| Revolutions / km | 502 | 493 | — |
Similar diameter sizes
Compared with your current 225/45R17, not the new tire. Up to eight nearest diameter matches within ±3% from a bounded nearby candidate grid. Availability is not checked.
These are mathematical diameter matches, not vehicle-specific fitment recommendations. Verify wheel width, offset, clearances, load/speed ratings and manufacturer requirements before changing tire size.
- 205/55R16631.9 mm diameter−0.4% vs current
- 205/50R17636.8 mm diameter+0.4% vs current
- 225/40R18637.2 mm diameter+0.5% vs current
- 225/50R16631.4 mm diameter−0.5% vs current
- 215/40R18629.2 mm diameter−0.8% vs current
- 245/35R18628.7 mm diameter−0.9% vs current
- 245/40R17627.8 mm diameter−1.0% vs current
- 235/50R16641.4 mm diameter+1.1% vs current
Keep this comparison
Review the text to copy or share
BuzCalculator — Tire Size Comparison 225/45R17 → 235/40R18 Diameter difference: +1.7% Width change: +10.0 mm Sidewall change: −7.3 mm Overall diameter change: +10.9 mm Theoretical radius / ride-height contribution: +5.5 mm 60 mph indicated ≈ 61.0 mph modeled actual Within the common ±3% diameter guideline. Diameter similarity does not guarantee vehicle fitment or suitability. Nominal geometry only; assumes calibration to the current tire. Actual loaded dimensions and speedometer calibration may differ.
Copy comparison contains the summary above. Share and Copy link include only the two validated tire codes in a link; units are not saved. Nothing is sent to a sharing destination until you choose one. Opening a shared link sends its URL to the site as part of normal page loading.
About this calculator
Compare tire diameter, width, sidewall and circumference. Estimate speedometer differences and explore nominal alternatives without a fitment guarantee.
Compare two tire codes to see the change in diameter, sidewall, width and modeled speed. Explore nominal dimensions instantly, without entering a vehicle or creating an account.
How to use the Tire Size Calculator
Enter the three numbers from your current tire and the new size. Valid edits update the comparison immediately. Millimeters/Inches changes dimensional output; MPH/KM/H changes the standard speed examples. Neither toggle changes the tire codes or geometry. Reset restores 225/45R17 and 235/40R18, millimeters and mph.
Incomplete or invalid entries stay editable and pause the comparison. Whole-number widths of 125–355 mm, aspect ratios of 25–85% and rim diameters of 12–24 inches are supported. These are project input bounds, not a catalog of available tires. Flotation sizes, fractional rim diameters and load/speed ratings are not parsed.
How tire sizes work
In 225/45R17, 225 is nominal section width in millimeters, 45 is sidewall height as a percentage of that width, R means radial construction, and 17 is rim diameter in inches. The aspect ratio is not a height in millimeters, and R does not mean radius. Load index and speed rating are separate markings.
Tire size formula and methodology
For width W in mm, aspect ratio A in percent and rim diameter R in inches: sidewall = W × A / 100; rim diameter = R × 25.4; overall diameter = rim diameter + 2 × sidewall; circumference = π × overall diameter. One inch is exactly 25.4 mm.
Revolutions per mile = 1,609,344 ÷ circumference in mm; revolutions per kilometer = 1,000,000 ÷ circumference in mm. These are ideal geometric values, not measured loaded rolling revolutions. Inch outputs divide millimeters by 25.4. Calculations retain full precision; dimensions display to one decimal in mm or two in inches, percentages and speeds to one decimal, and revolutions to whole numbers.
How diameter difference is calculated
Diameter change = new diameter − current diameter. Percentage change = (new diameter ÷ current diameter − 1) × 100. Positive means larger; negative means smaller. Reversing current and new changes the percentage denominator, so the percentages are not exact opposites.
Theoretical radius change = diameter change ÷ 2. It describes the static contribution from tire geometry alone, not a guaranteed change in ground clearance. Load, tire deflection and the vehicle can change the actual result.
How tire size affects speedometer readings
Modeled actual speed = indicated speed × new diameter ÷ current diameter. The circumference ratio gives the same result because π cancels. A larger new tire gives a higher modeled speed for the same indicated speed; a smaller one gives a lower speed.
The table assumes calibration to the current size. It does not correct factory speedometer bias or measure real speed. MPH examples are 30, 50, 60 and 70; KM/H examples are 50, 80, 100 and 120. Switching units selects those examples rather than converting each row into the other unit.
What the ±3% diameter guideline means
The Size Difference Check labels an absolute diameter change of at most 3% as within the common guideline, including exactly −3% and +3%. A greater change is outside. Classification uses the unrounded geometry, not the displayed one-decimal percentage; a value just above 3% may display as 3.0% while remaining outside.
This is a comparison convention, not a law, universal safety threshold, manufacturer approval or recommendation for a particular vehicle.
Why diameter similarity does not guarantee fitment
Diameter similarity does not guarantee vehicle fitment or suitability. This calculator does not know your make/model, wheel width or offset, suspension geometry, brake or wheel-well clearances, load/speed ratings, manufacturer requirements or legal requirements. It cannot check rubbing while steering or during suspension compression. Verify the specific tire and wheel combination against vehicle and tire manufacturer information.
How similar diameter sizes are selected
Candidates use widths 145–335 mm in 10 mm steps, aspect ratios 30–80% in 5-point steps and rims 14–22 inches in whole-inch steps. Only candidates within 20 mm of current width, 15 aspect-ratio points and 1 rim inch are considered.
The same geometry model filters to ±3% of the current tire diameter, excludes the current code and returns at most 8 matches. Ranking uses absolute diameter difference; exact ties use width, then aspect ratio, then rim diameter ascending. This bounded project search is not exhaustive and does not establish availability, wheel compatibility or fitment.
Worked example
Worked example: 225/45R17vs 235/40R18
The current sidewall is 225 × 45 / 100 = 101.25 mm. Its overall diameter is 431.8 mm + 2 × 101.25 mm = 634.3 mm (24.97 in).
The new sidewall is 235 × 40 / 100 = 94.00 mm. Its diameter is 645.2 mm (25.40 in). The difference is 10.9 mm, or approximately 1.72% (+1.7% in the summary).
Half that difference gives a theoretical radius change of 5.45 mm (0.21 in). At 60 mph indicated, 60 × 645.2 ÷ 634.3 gives approximately 61.03 mph, displayed as 61.0 mph. All example outputs are generated by the same model as the interactive comparison.
Assumptions, limitations and privacy
Nominal unloaded geometry is an approximation. Manufacturer construction, measuring rim, pressure, load and tread wear can affect physical dimensions and rolling behavior. No tire inventory, vehicle database, fitment approval or real speed measurement is used. The diagram is a dimensional comparison, not a suspension or clearance simulation.
Edits and calculations run locally in tab memory. No cookies, browser storage, analytics or calculation API requests are added. Copy comparison writes plain text only after your click. Share and Copy link can expose the two validated tire codes in a URL on this calculator page; no other state is encoded. Opening that link sends the URL to the hosting site in the normal page request. Reset removes query state without navigating away. Units are not saved.
Tire Size Calculator FAQ
How do I read a tire size such as 225/45R17?
225 is nominal width in millimeters, 45 is sidewall height as a percentage of that width, R means radial construction and 17 is rim diameter in inches. Enter only those three numbers in the labeled controls.
How is overall tire diameter calculated?
Multiply width by aspect ratio divided by 100 to find one sidewall. Add twice that height to the rim diameter converted from inches to millimeters. The rim alone is not the overall tire diameter.
What does the diameter percentage difference mean?
It measures the new diameter relative to the current one. A positive value means larger and a negative value means smaller. Equal nominal diameters give zero even if width, sidewall and rim dimensions differ.
Is a tire within 3% guaranteed to fit my vehicle?
No. The inclusive ±3% check is only a diameter comparison convention. It does not establish clearance, load capacity, manufacturer approval, legality or suitability for your vehicle.
How does changing tire size affect the speedometer?
The model multiplies indicated speed by new diameter divided by current diameter. Larger tires travel farther per revolution, so modeled actual speed is higher. This assumes the current size is the calibration reference and does not correct existing speedometer error.
Why does the calculator show half the diameter change as a ride-height contribution?
Radius is half of diameter. That geometric difference is the theoretical static contribution, not a promise that ground clearance changes by exactly that amount. Tire deflection and vehicle conditions are not modeled.
Are similar diameter sizes available or guaranteed to fit?
No. They are generated mathematical matches from a limited nearby grid, not an inventory or vehicle recommendation. Verify availability, wheel compatibility, clearances, ratings and manufacturer requirements separately.
Does this calculator account for wheel width or offset?
No. Tire section width is not wheel width, and rim diameter does not describe offset or brake clearance. Neither the diagram nor the percentage check can resolve those missing measurements.
Can I use km/h and inches without changing the tire codes?
Yes. KM/H selects indicated-speed examples of 50, 80, 100 and 120; Inches converts dimensional output from the same millimeter model. Standard tire-code inputs retain their original mm, percent and inch units.
Why can actual tire dimensions differ from these results?
The calculation uses nominal size markings rather than measurements of a specific tire. Tire construction, measuring rim and operating conditions can change physical size or rolling circumference. Consult the exact tire manufacturer specifications for close comparisons.
Sources and review
Reviewed September 26, 2026. Continental’s tire-marking guide (opens in a new tab) explains the mixed-unit size notation. Tire Rack’s dimension guide (opens in a new tab) describes nominal geometry and its difference from manufacturer specifications. Its tire-selection guide (opens in a new tab) discusses a 3% diameter-change convention for cars and vans; that contextual guidance is not universal vehicle approval.
The input limits, candidate grid, ranking and display precision are project choices. These static references do not provide a vehicle-specific fitment database, and no source is fetched during calculation.