Tire Size Comparison Calculator
Two sizes, one clear comparison. Explore nominal tire dimensions and the estimated effect on your speedometer.
Side by side, to the same scale
Nominal dimensions
| Dimension | Original | New | Change |
|---|
Estimated speed at this indication
Nominal size comparison only. A small percentage difference does not establish compatibility. Confirm the exact tire, wheel, load and speed ratings, clearance, and vehicle-specific limits with the manufacturers or a qualified tire professional. The speed estimate uses nominal geometry, not measured rolling circumference.
Compare the dimensions behind the tire code
A different wheel or tire size changes more than the number on the sidewall. This calculator puts two metric passenger tire sizes side by side so you can see how nominal width, sidewall height, overall diameter and circumference compare. The drawing uses one shared scale for both tires and their rims.
Use this as a first geometry check when discussing a replacement with a tire professional. It is not a tire catalog, vehicle fitment database or approval to install a particular combination. It does not estimate grip, ride quality, fuel economy or EV range.
How to compare two tire sizes
- Start with the original size. Enter its section width in millimeters, aspect ratio as a percentage, and rim diameter in inches. For 225/65R17, enter 225, 65 and 17. Confirm the vehicle-approved specification in the owner's manual or tire information placard.
- Enter the size you want to compare. Use the same three fields. The sidewall code does not give a wheel's width, offset, load rating or the tire's required inflation pressure.
- Compare the dimensions. The table shows millimeters and inches. Every change is new minus original; the percentage uses the original overall diameter as its baseline.
- Optionally check a speedometer scenario. Enter an indicated speed and select mph or km/h. The selector labels the number you enter; it does not convert it. Clear the speed field to leave only the geometry comparison.
- Verify an actual replacement separately. Use manufacturer specifications and vehicle-specific requirements before buying or fitting tires. Swapping the two sizes changes the percentage baseline and the assumed original calibration.
How tire dimensions are calculated
Rim diameter in mm = rim diameter in inches × 25.4
Overall diameter = rim diameter in mm + 2 × sidewall height
Geometric circumference = π × overall diameter
Diameter change (%) = (new diameter ÷ original diameter − 1) × 100
Estimated speed = indicated speed × new diameter ÷ original diameter
Divide millimeters by 25.4 to convert them to inches. The diameter and circumference have the same percentage change because circumference is calculated as π times diameter. This is a nominal geometric model, not a measurement of the tire's loaded rolling circumference.
Example: 225/65R17 vs 235/60R18
The original 225/65R17 has a nominal sidewall height of 146.25 mm and diameter of 724.30 mm. The 235/60R18 has a sidewall height of 141.00 mm and diameter of 739.20 mm. That is a 14.90 mm increase in diameter, or about 2.06%, even though its sidewall is 5.25 mm shorter.
At an indicated 60 mph, the model estimates approximately 61.23 mph with the new size, assuming the speedometer was accurate for the original size and no recalibration was made. These are arithmetic examples, not a recommendation to fit either tire to any vehicle.
What the percentage cannot tell you
A diameter comparison cannot prove that a wheel and tire assembly is suitable. Check the tire manufacturer's approved rim width and contour, the wheel's ratings, required tire load capacity and speed rating, prescribed inflation, and clearance throughout steering and suspension travel. A wider tire or different wheel offset can affect clearance even if the calculated diameter barely changes.
Size changes may affect speedometer and odometer readings, ABS, traction control and other vehicle systems. AWD and four-wheel-drive vehicles may require closely matched tire circumferences; follow the exact manufacturer's limits and replacement instructions. A small percentage difference is not an approval threshold.
Nominal dimensions and calculation limits
Inputs are limited to a section width of 50–500 mm and aspect ratio of 10–100%, both whole numbers, with rim diameter from 8–32 inches. The optional speed accepts 0–400 in the selected unit. These are software bounds, not a list of available tires or recommended driving speeds. Metric passenger radial geometry is the only size format modeled; load indexes, speed symbols, P/LT/XL markings and specialty tire systems are not evaluated.
Results are rounded to two decimal places only for display. Calculations use the unrounded values. Real dimensions depend on construction and measuring conditions; inflation, load, wear and speed can affect rolling dimensions. A geometry-based speed estimate also ignores pre-existing instrument error and slip. Confirm actual tire specifications rather than treating these figures as measured data.
Tire size comparison FAQ
What does 225/65R17 mean?
225 is the nominal section width in millimeters. 65 means the nominal sidewall height is 65% of that width. R indicates radial construction, and 17 is the wheel rim diameter in inches. Section width is not the same as tread width. This calculator uses those three numeric dimensions.
Does a small diameter difference mean the new tire will fit?
No. A small percentage difference does not establish compatibility, and this tool does not apply a universal safe percentage. Confirm approved rim widths and contours, tire and wheel load limits, required speed ratings, inflation, clearance, and vehicle-specific instructions with the manufacturers or a qualified tire professional.
Why is the speed estimate different with larger tires?
With unchanged calibration, a larger rolling circumference covers more distance per wheel revolution. This model uses the nominal diameter ratio as an approximation: estimated speed equals indicated speed multiplied by new diameter divided by original diameter. It assumes the original calibration was accurate and ignores existing instrument error, slip, wear, and actual rolling-dimension differences.
Does this work for AWD and four-wheel-drive vehicles?
It can compare the nominal geometry, but it cannot approve an AWD or four-wheel-drive replacement. Some vehicles have strict circumference-matching requirements, and even small mismatches can damage the drivetrain. Follow the exact vehicle manufacturer instructions, including tire replacement and matching requirements.
Can I enter flotation, LT, P-metric or motorcycle sizes?
This tool models only the three-number geometry of metric radial passenger tire sizes such as 225/65R17. It does not parse complete sidewall markings or interpret P, LT, XL, load indexes or speed symbols. It does not support flotation sizes such as 35×12.50R17, motorcycle fitment, or specialty tire systems. Even when numbers can be entered, that does not confirm a tire exists or is appropriate.
Why might manufacturer dimensions differ from the result?
Sidewall markings describe nominal dimensions. Actual dimensions depend on the tire design, measuring rim, inflation, load and wear. Measured rolling circumference is not simply the unloaded diameter multiplied by pi, and revolutions per mile can also vary with speed. Use the exact tire manufacturer specifications when evaluating a real replacement.
Sources and further reading
- Michelin: tire markings explained explains nominal section width, aspect ratio and rim diameter.
- Michelin tire glossary distinguishes overall diameter, rolling circumference and revolutions per mile.
- USTMA: Care and Service of Passenger and Light Truck Tires, printed pages 33–37 (PDF pages 37–41), covers replacement, rim checks and vehicle-specific limits.
Sources reviewed October 5, 2026. Independent educational calculator; not affiliated with tire or vehicle manufacturers.