Use this free Tire Size Calculator to compare two metric tire sizes. Calculate overall diameter, sidewall height, circumference, revolutions per mile, ground-clearance change and speedometer difference.
How the Tire Size Calculator works
The Tire Size Calculator converts the metric width and aspect ratio into sidewall height, adds both sidewalls to the wheel diameter, and compares the resulting tire diameters. It also estimates circumference, revolutions per mile and the speedometer change caused by the new diameter.
Tire codes describe nominal dimensions. Actual mounted dimensions can vary by brand, model, wheel width, inflation pressure, load and tread wear. Use calculated results for preliminary comparison and verify final fitment against manufacturer specifications.
Tire diameter formula
Sidewall height in millimeters = tire width × aspect ratio ÷ 100. Convert sidewall height to inches by dividing by 25.4. Overall diameter equals wheel diameter plus two sidewalls.
Overall diameter = wheel diameter + 2 × sidewall height in inches. Circumference equals diameter multiplied by π. Approximate revolutions per mile equal 63,360 inches divided by circumference in inches.
Worked tire-size example
A 205/55R16 tire has a sidewall height of 205 × 0.55 = 112.75 mm, or about 4.44 inches. Overall diameter is approximately 16 + 2 × 4.44 = 24.88 inches. A 225/50R17 tire has a sidewall height of 112.5 mm and a diameter of approximately 25.86 inches. The new tire is therefore about 3.9% larger in diameter.
Speedometer difference
A speedometer generally estimates road speed from wheel rotation. A larger tire travels farther per revolution, so actual speed becomes higher than the indicated speed when the vehicle is not recalibrated. A smaller tire travels less distance per revolution, making actual speed lower.
Actual speed = indicated speed × new diameter ÷ current diameter. If the new diameter is 3% larger, an indicated 60 mph corresponds to approximately 61.8 mph. Vehicle systems vary, so this is a mathematical estimate rather than a calibration guarantee.
Ground-clearance change
Only half of a diameter difference affects ride height because the axle sits near the tire center. Ground-clearance change equals the new diameter minus current diameter, divided by two. A tire that is one inch larger in diameter raises theoretical axle height by about half an inch.
What diameter difference is acceptable?
Many fitment comparisons try to remain within roughly 3% of the original overall diameter, but this is not a universal approval rule. Some vehicles allow less, and some modifications require professional inspection, wheel changes, recalibration or suspension work. Consult the vehicle manufacturer, tire placard and a qualified installer.
Other fitment factors
- Wheel diameter and width: the tire must be approved for the wheel dimensions.
- Wheel offset: incorrect offset can cause suspension or body interference.
- Load index: the tire must support the vehicle’s required load.
- Speed rating: use a rating appropriate for the vehicle and local requirements.
- Clearance: check turning, braking, suspension compression and snow-chain space.
- Drivetrain: all-wheel-drive vehicles may require very closely matched rolling circumferences.
Why revolutions per mile matter
Revolutions per mile affect speed calculations, odometer readings, gearing and some electronic systems. Larger tires have fewer revolutions per mile; smaller tires have more. Real published revolutions-per-mile figures may differ slightly from geometric estimates because tires deflect under load.
The Tire Size Calculator is designed for transparent comparison. For official tire safety guidance, review the NHTSA tire safety information. Recheck every value entered in the Tire Size Calculator before choosing a replacement. The Tire Size Calculator cannot confirm physical clearance or manufacturer approval.
Frequently asked questions about the Tire Size Calculator
Can I install any tire with a similar diameter?
No. Diameter is only one requirement. Wheel width, rim diameter, offset, load capacity, clearance, speed rating and manufacturer approval must also be checked.
Does a larger tire increase ground clearance?
It can. The theoretical increase is half the diameter increase, but suspension, pressure and loaded tire radius influence actual clearance.
Will changing tire size affect MPG?
It may. Weight, rolling resistance, aerodynamics and effective gearing can affect fuel economy. An odometer error can also distort calculated MPG unless corrected.
Why does the sidewall use a percentage?
The aspect ratio expresses sidewall height as a percentage of tire width. A 55-series sidewall is 55% of the nominal section width.
Are calculated dimensions exact?
No. They are nominal geometric estimates. Check the tire manufacturer’s specification sheet for measured diameter, approved rim widths and revolutions per mile.
