Tire Size Calculator
Compare tire dimensions, speedometer difference and ground clearance; visualize two sizes and find nominally similar alternatives.
Nominal sizing only—not a vehicle fitment or safety approval. Terms and Conditions
W = nominal width (mm), A = aspect ratio (%), R = wheel diameter (in); flotation sizes use their stated nominal diameter.
Enter the tire sizes
Enter metric or inch-designated radial sizes; results update automatically.
Tire comparison results
Nominal geometry; not an approved replacement or fitment determination.
Dimensions and differences
- Comparison—
Show calculation stepsDimensions, speedometer correction and assumptions
- Enter valid sizes to view steps.
Tire size visual comparison
Both tire profiles use the same scale; rim circles are shown inside each tire.
Nominal dimensions shown at the same scale.
Method, sources and assumptions
What this calculation can and cannot determine.
Metric sizes: nominal sidewall = section width × aspect ratio / 100; nominal overall diameter = wheel diameter + two sidewalls. Inch-designated flotation tires supply nominal outside diameter and width directly. 25.4 mm = 1 in; 1 mi = 1,609,344 mm; 1 mi = 1.609344 km.
- Dimensions and revs per mile are geometric estimates, not measurements of a loaded rolling tire. Actual tire model, inflation, rim width, wear, load and calibration can change results.
- Speed and distance corrections assume the vehicle was calibrated accurately for the original rolling circumference. Verify load index, speed rating, wheel fit, clearances and approved replacement sizes using manufacturer data and the vehicle placard.
- Alternative sizes are mathematical candidates only. A small diameter difference does not establish availability, safety, drivetrain compatibility or vehicle fitment.
Calculator guide
How to Compare Tire Sizes
A tire size calculator compares the nominal dimensions of your existing and proposed tires. Enter the original and new size markings in the calculator above to see the percentage change in overall diameter, section width, estimated speedometer reading, and ground clearance. Metric sizes derive diameter from width, aspect ratio, and wheel diameter; inch-designated flotation sizes state a nominal diameter directly.
Start with the tire size specified for your vehicle, not simply a size that happens to be installed now. The calculation answers how the two size markings compare; it cannot establish that the new tire fits your wheel, clears your vehicle, or meets its load and speed requirements.
- Enter
- Original and replacement tire sizes
- Main result
- Nominal outside-diameter difference (%)
- Before fitting
- Confirm vehicle, wheel, load, and clearance requirements
How to Use the Tire Size Calculator
Use the original and replacement size markings to compare nominal tire dimensions. The first-load 225/65R17-to-235/65R17 example is illustrative, not a replacement recommendation for your vehicle.
Enter both tire sizes
Enter the complete original and proposed size in their labeled fields. For example, use 225/65R17 for a metric tire or 33×12.50R17 for a flotation tire. Put the original size first: reversing the two sizes reverses the percentage-change sign and speed correction.
Read the comparison before choosing a size
Start with the nominal diameter difference, then examine section-width and sidewall changes, estimated actual speed, and geometric clearance change. The scaled tire profiles illustrate the relative diameters; they do not represent your vehicle’s fender or suspension clearances.
Use Advanced Options only when needed
Adjust indicated speed or distance to estimate the corresponding actual value for the proposed tire. Switching the display selector between inches and millimeters also switches speed and distance between mph/miles and km/h/kilometers while preserving the entered physical quantities. To search for alternatives, set the maximum nominal diameter difference and choose the desired wheel diameter; selecting a candidate loads it as the proposed tire.
How to Read Tire Size Markings
In a marking such as 225/65R17 102H, the first three numbers describe nominal tire geometry; the remaining service description identifies load and speed ratings. The tire’s overall diameter is not 17 inches: 17 inches is the wheel diameter at the tire’s bead seat.
- 225 — nominal section width
- The first number is width in millimeters, measured across the tire’s section rather than its tread contact area. Actual mounted section width depends in part on rim width.
- 65 — aspect ratio
- The sidewall height is nominally 65% of the section width: 225 × 0.65 = 146.25 mm. The number 65 is a percentage, not a 65 mm height.
- R17 — construction and rim diameter
- R indicates radial construction; 17 specifies the compatible wheel bead-seat diameter in inches. Changing from R17 to R18 requires a different wheel.
- 102H — load index and speed symbol
- These service markings are not included in the nominal diameter formula. Confirm the proposed tire’s load capacity and speed rating against vehicle and tire-manufacturer requirements.
Metric versus flotation sizes: LT285/70R17 uses a metric section width and aspect ratio. 33x12.50R17 instead states a nominal 33-inch outside diameter, 12.50-inch width, and 17-inch wheel diameter. The two sizing systems are not interchangeable on the strength of their names alone.
Tire Size Calculation Formulas
For a metric radial tire, calculate one nominal sidewall height, double it, and add the wheel diameter after converting inches to millimeters. The calculator applies this geometry independently to the original and replacement sizes, then compares their unrounded diameters.
Nominal sidewall height and outside diameter
Sidewall height equals section width multiplied by aspect ratio divided by 100. Outside diameter equals wheel diameter in millimeters plus two sidewall heights.
Diameter comparison and idealized speed correction
Divide the new nominal diameter by the original to obtain the size ratio. Multiply the indicated speed by that ratio to estimate actual speed, assuming the vehicle was accurately calibrated for the original tire’s rolling distance.
Additional geometric relationships
Ideal geometric circumference equals pi times diameter. The estimated change in axle height is half the nominal diameter change; actual loaded clearance can differ.
- \(W\)
- Section widthThe width number in a metric tire-size marking.
- \(A\)
- Aspect ratioThe sidewall-height percentage of nominal section width.
- \(R\)
- Wheel diameterBead-seat diameter stated in the tire-size code.
- \(H\)
- Nominal sidewall heightOne sidewall, calculated from the metric size code.
- \(D\)
- Nominal overall diameterOuter tire diameter derived from the size designation.
- \(D_1,D_2\)
- Original and replacement diametersNominal outside diameters before and after the proposed size change.
- \(C\)
- Geometric circumferenceIdeal circumference computed from nominal overall diameter.
- \(\Delta h\)
- Estimated height changeHalf the nominal outside-diameter difference.
- \(V_{\mathrm{actual}},V_{\mathrm{indicated}}\)
- Actual and indicated speedsRoad speed estimate and speedometer reading under the stated calibration assumption.
Use the exact conversion 1 inch = 25.4 mm. For geometric revolutions per mile, divide 1,609,344 mm by the ideal circumference expressed in millimeters. This is a geometric estimate, not the measured revolutions per mile of a loaded tire.
Worked Example: 225/65R17 vs. 235/65R17
Suppose your original size is 225/65R17 and you are comparing it with 235/65R17. The wheel diameter and aspect ratio stay the same, but increasing section width also increases the nominal sidewall height.
Calculate the original and new diameters
The original and replacement diameters are approximately 28.52 and 29.03 inches.
Compare sizes and correct indicated speed
Result
+1.79% nominal diameter; approximately 61.08 mph actual at 60 mph indicated
The new tire is 13.0 mm (0.51 in) larger in nominal diameter. Its geometric radius is 6.5 mm (0.26 in) greater, but this does not establish actual vehicle fitment or loaded clearance.
What the Tire Size Difference Means
A positive diameter percentage means the replacement tire is nominally taller; a negative value means it is shorter. The result is a relative geometric change, not a score for whether a tire is suitable for your vehicle.
Speedometer and odometer
With a larger replacement rolling circumference and unchanged original-size calibration, the speedometer and odometer tend to under-report actual speed and distance. The tool estimates this effect using nominal diameter, while actual tire rolling behavior and vehicle calibration may differ.
Ground clearance
In ideal geometry, a 1-inch increase in outside diameter raises the wheel center by 0.5 inch. Check clearance at the relevant component and allow for loaded tire deflection; a nominal radius gain does not mean every underbody point gains that amount.
Sanity check
Identical size codes must produce a zero nominal difference. If width increases while aspect ratio and wheel diameter remain unchanged, the calculated sidewall height and diameter must also increase. If a result seems reversed, check which size was entered as original.
| Indicated speed (mph) | Estimated actual speed (mph) |
|---|---|
| 30 | 30.54 |
| 45 | 45.81 |
| 60 | 61.08 |
| 70 | 71.26 |
For an indicated distance of 100 miles in this example, the corresponding idealized actual distance is about 101.79 miles. Real-world speedometer or odometer correction is more reliably evaluated with the exact tires’ published rolling characteristics and the vehicle’s calibration data.
Finding Alternative Sizes and Checking Fitment
A mathematically close alternative is a candidate for further checking, not an approved replacement. The calculator’s alternative-size finder filters possible metric combinations by nominal diameter tolerance and selected wheel diameter; it does not confirm that every combination is manufactured, stocked, or suitable for your vehicle.
Plus sizing: a bigger wheel, similar tire diameter
Changing from 225/65R17 to 225/60R18 raises the wheel bead-seat diameter by 1 inch while reducing sidewall height. Their calculated outside diameters are 724.3 and 727.2 mm, only about 0.40% apart. The 18-inch tire still requires an 18-inch wheel; brake clearance and wheel fit also need checking.
Metric versus nominal 33-inch sizing
LT285/70R17 has a calculated nominal diameter of 830.8 mm, or 32.71 inches. A 33×12.50R17 flotation tire designates a nominal 33-inch diameter and a much wider nominal section. Similar nominal height does not mean equal width, load capacity, or actual measured diameter.
Diameter tolerance is a search filter, not a safety standard. For example, a candidate within 2% of the original diameter can still be too wide for the current rim or vehicle. A size appearing in the alternative finder also does not establish that a manufacturer offers it.
Before selecting a replacement
- Start with the vehicle specification. Find the tire size and loading information on the driver’s door-edge placard or in the owner’s manual. See NHTSA’s tire-size guidance for the recommended selection approach.
- Check the wheel. Tire and wheel bead-seat diameters must match; the exact tire model must also permit the installed rim width. Wheel offset, spoke-to-brake clearance, and mounting specifications are separate checks.
- Check load and service requirements. Confirm load index, speed rating, load range where applicable, and inflation/load requirements against vehicle and tire-manufacturer documentation.
- Check all relevant clearances. Evaluate inner suspension components, outer fender, steering lock, and suspension travel. A nominal width difference is not a direct measured rubbing clearance.
- Check all driven wheel positions. For an AWD or 4WD vehicle, consult the vehicle manufacturer’s requirements for rolling-circumference and tread-depth differences instead of assuming one generic acceptable percentage.
Why Real Tire Dimensions May Differ
The size designation describes nominal geometry, while an actual tire has manufacturer-specific dimensions and changes shape under load. This matters most when estimating tight clearance or accurate revolutions per mile.
Measuring rim and mounted section width
The same tire can measure wider or narrower on different rim widths. Consult the exact model’s published section width, specified measuring rim, and approved rim-width range rather than using the nominal width difference as a guaranteed clearance change. See Tire Rack’s specification guide.
Loaded rolling dimensions
The tire flattens at the road contact patch and its tread and sidewall deform as it rolls. Therefore geometric circumference divided into one mile does not equal precise operating revolutions per mile. Manufacturer-published revolutions per mile are a more relevant tire-specific reference when available.
Inflation, load, and wear
Vehicle loading, inflation, and tread depth change tire deflection or the effective rolling geometry. Compare tires under relevant operating conditions and follow the vehicle manufacturer’s specified cold inflation pressures; do not use a tire sidewall’s maximum pressure as a universal vehicle setting.
Nominal size versus actual model
Two tire models bearing the same size marking may have different published overall diameters, section widths, and rolling specifications. For close-fit applications, use the manufacturer’s model-specific measurements and confirm actual vehicle clearances.
Calculation Limits and Final Checks
This tool is a nominal dimension-comparison calculator. It does not test a physical vehicle and cannot infer the manufacturer’s approved replacement tires from the size code alone.
Ideal circumference assumption
Estimated speed, distance, and geometric revolutions per mile are derived from nominal outside-diameter ratios. They assume the original calibration is accurate and the rolling-distance ratio follows the nominal diameter ratio.
Flotation designations
In 33×12.50R17, the 33-inch diameter and 12.50-inch width are nominal designations, not a measurement of every manufactured tire bearing that marking. Confirm actual diameter and mounted width in the tire manufacturer’s specifications.
Clearance interpretation
A geometric radius increase estimates axle-height change, not guaranteed clearance at every chassis, body, or suspension component. Dynamic rubbing during steering and suspension travel cannot be established from tire size alone.
Availability and approval
Alternative-size results are generated mathematically. Verify that the tire is available, approved for the intended rim width, has appropriate ratings, and is recommended or permitted for the specific vehicle.
Technical Sources and Verification
The size arithmetic follows the dimensions stated by conventional metric and flotation size designations; replacement-tire advice and measurement limitations are supported by the technical sources below. The worked example was checked independently by diameter difference and inverse speed calculation.
- NHTSA — Tire Safety Ratings and Awareness — guidance on vehicle tire-size selection, the owner’s manual, the Tire and Loading Information label, and proper tire inflation.
- Tire Rack — How Do I Check My Tire Specs? — published rim-width ranges, measuring-rim width, section width, overall diameter, and the difference between ideal geometry and real revolutions per mile.
- Tire Rack — Do All Four Tires Need to Match on AWD or 4WD? — tire circumference, tread-depth matching, and drivetrain considerations for applicable vehicles.
Tire Size Questions
Can I replace 225/65R17 tires with 235/65R17 tires?
The latter size is nominally 10 mm wider and 13 mm taller overall, a diameter increase of about 1.79%. The dimensional comparison alone cannot establish that the proposed tire is an approved replacement. Check the vehicle manufacturer’s recommended tire sizes, the tire’s ratings, the rim-width range, and available clearance.
How much taller is a tire when its width increases?
If aspect ratio and wheel diameter are held constant, each 10 mm increase in nominal section width adds 10 × aspect ratio ÷ 100 mm to the nominal height of one sidewall. At a 65% aspect ratio, that is 6.5 mm per sidewall and 13 mm in overall diameter.
What metric tire size equals a 33-inch tire?
There is no unique metric equivalent. For example, LT285/70R17 calculates to about 32.71 inches in nominal outside diameter but is narrower than a 33×12.50R17 designation. Compare the exact tire models’ published measurements, load ratings, and wheel requirements rather than selecting a replacement from nominal height alone.
Will moving to a larger wheel make the overall tire taller?
Not necessarily. A larger rim can be paired with a tire having shorter sidewalls to retain a similar outside diameter. For example, 225/65R17 and 225/60R18 have calculated nominal diameters of 724.3 and 727.2 mm. The larger wheel still needs vehicle and tire compatibility verification.
Does a small diameter difference guarantee no tire rubbing?
No. Tire section width, wheel offset, actual measured tire dimensions, steering position, and suspension travel all affect clearance. A tire with the same nominal outside diameter can still contact the fender or suspension if its width or wheel position changes.
Why does calculated revolutions per mile differ from a tire specification sheet?
Dividing a mile by the circumference of a nominal unloaded circle gives a geometric estimate. A real rolling tire deforms under load, and manufacturers may determine revolutions per mile through testing or other established methods. Use the exact tire model’s published rolling specification when the distinction matters.