How it works
In 225/65R17, 225 is nominal section width in millimeters, 65 is sidewall height as a percentage of width, R means radial construction, and 17 is rim diameter in inches. Section width is not wheel width. In 33x12.50R17, 33 is overall diameter and 12.50 is section width, both in inches. Flotation sidewall height is half the difference between overall diameter and rim diameter. Calculations keep full precision. Ordinary results use up to three decimal places; small results use up to three significant digits. Very small or large results use scientific notation: 1E-8 means 0.00000001. Displayed digits do not indicate measurement accuracy.
The formulaMetric sidewall (mm) = width x aspect ratio / 100
Diameter (mm) = 2 x sidewall + rim (in) x 25.4
Circumference = pi x diameter
Revs/mile = 1,609,344 / circumference (mm)
Revs/km = 1,000,000 / circumference (mm)
Diameter change (%) = (new / original - 1) x 100
Actual speed = indicated speed x new / original diameter
Put it into practice
EXAMPLE 01225/65R17 has a 146.25 mm sidewall and a 724.3 mm (28.516 in) diameter. Its nominal circumference is 2,275.456 mm, or about 707.262 theoretical revolutions per mile. Changing to 235/65R17 adds 13 mm (1.795%) to diameter. At an indicated 60 mph, estimated actual speed becomes 61.077 mph.
EXAMPLE 0233x12.50R17 has an 838.2 mm diameter, 317.5 mm section width and 203.2 mm sidewall height. Comparing it with the same size gives zero diameter change and no calculated speedometer effect.
Good to know
Does a small percentage change mean the tire fits?
No percentage is a universal safe-fit rule. This tool has no vehicle lookup, approved-size database or wheel-offset inputs. It cannot predict rubbing, clearances or compatibility. Consult the vehicle placard/manual and a qualified tire professional for approved wheel dimensions, load and speed ratings, inflation and drivetrain requirements. Changes can affect gearing, braking, ABS and stability systems.
Why might measured dimensions or revolutions differ?
These are nominal geometric estimates, not measured loaded rolling circumference. Tire model, wheel width, pressure, load and wear affect real dimensions and rolling behavior. Use the tire manufacturer’s specifications and vehicle requirements for decisions.
Why does a larger tire mean a higher actual speed?
A larger circumference travels farther per revolution. If the speedometer remains calibrated to the original size, the model predicts a higher actual speed for the same indicated reading. A smaller tire predicts a lower speed. Existing speedometer error and recalibration are not included.
Which formats and ranges are supported?
Metric radial sizes accept no prefix, P or LT, widths 100-500 mm, aspect ratios 20-100%, and rims 10-30 inches. Flotation sizes accept diameters 20-60 inches, widths 5-25 inches and rims 10-30 inches; overall diameter must exceed rim diameter. Decimal flotation dimensions and rims accept up to two decimal places. Case and spaces around separators are accepted; spaces inside numbers are not. These are input limits, not an availability or compatibility check. ZR, bias-ply, commercial C suffixes, old numeric sizes and service descriptions are unsupported.
Can I compare P and LT tires or use this for AWD?
The tool compares geometry only. P and LT tires are not automatically interchangeable, and AWD/4WD vehicles may have strict matching requirements. Follow vehicle and tire manufacturer guidance even when calculated dimensions match.