Calculator Inputs
Formula Used
Sidewall Height = Section Width × Aspect Ratio ÷ 100
Unloaded Diameter = Rim Diameter × 25.4 + 2 × Sidewall Height − 2 × Tread Wear Loss
Static Radius = Unloaded Diameter ÷ 2
Loaded Radius = Static Radius − Loaded Deflection
Rolling Radius = Loaded Radius × Rolling Factor ÷ 100
Rolling Circumference = 2 × π × Rolling Radius
Revolutions Per Kilometer = 1,000,000 ÷ Rolling Circumference
Vehicle Speed = Rolling Circumference × Wheel RPM × 60 ÷ 1,000,000
Actual Speed = Indicated Speed × New Rolling Radius ÷ Reference Rolling Radius
Example Data Table
| Tire Size | Deflection | Rolling Factor | Estimated Rolling Radius | Approx Revs Per Mile |
|---|---|---|---|---|
| 205/55R16 | 11 mm | 100% | 305.95 mm | 837.36 |
| 225/55R17 | 12 mm | 100% | 316.15 mm | 810.35 |
| 265/70R17 | 18 mm | 100% | 389.40 mm | 657.89 |
How To Use This Calculator
- Enter the tire section width from the tire code.
- Enter the aspect ratio shown after the slash.
- Enter the rim diameter shown after the R mark.
- Add tread wear loss if the tire is worn.
- Enter loaded deflection if you measured tire compression.
- Use 100% rolling factor for a basic estimate.
- Add wheel RPM to estimate vehicle speed.
- Add reference rolling radius to compare speedometer error.
- Click calculate, then download the result if needed.
Understanding Tire Rolling Radius
Tire rolling radius is the effective radius a tire uses while moving. It is not always the same as half of the unloaded tire diameter. A real tire bends under vehicle weight. Air pressure, load, tread wear, and speed also change the contact shape. For that reason, engineers often use rolling radius when studying speed, gearing, torque, and odometer behavior.
Why The Value Matters
A small radius change can affect many vehicle readings. Larger rolling radius makes one wheel turn cover more distance. Smaller rolling radius makes the wheel turn more often over the same road length. This matters for speedometer checks, transmission planning, electric vehicle range studies, tire replacement, and drivetrain comparisons. Racing teams also review the number before choosing a tire compound or diameter.
Geometry Behind The Calculator
The calculator begins with tire code dimensions. Section width and aspect ratio give sidewall height. Rim diameter is converted from inches to millimeters. Two sidewalls are added to the rim diameter. Tread loss is then removed from both sides. The result gives an estimated unloaded diameter and static radius.
Loaded And Rolling Effects
The next step accounts for tire compression. Deflection reduces the static radius because the tire flattens at the contact patch. A rolling factor can also be applied. It represents casing growth, load variation, and test assumptions. When exact laboratory data is missing, the estimate still gives a practical comparison between tire sizes.
Speed And Revolutions
After radius is known, circumference follows from two times pi times radius. Circumference shows how far the vehicle moves per wheel turn. The calculator converts that distance into revolutions per kilometer and mile. It also estimates vehicle speed from wheel RPM. If a reference radius is entered, the tool compares indicated speed with the new tire setup.
Practical Use
Use measured data when possible. Tire labels are nominal. Real dimensions vary by brand, rim width, pressure, and wear. For best results, measure loaded radius on level ground. Then compare it with the calculated output. The result is an estimate, not a legal calibration certificate. Keep inputs consistent. Do not mix loaded measurements with unloaded catalog values. Document assumptions before using results for gear selection, planning, or careful diagnostics.
FAQs
What is tire rolling radius?
It is the effective radius used when a tire rolls on the road. It includes tire compression, loading, and practical movement effects.
Is rolling radius the same as static radius?
No. Static radius is usually measured at rest. Rolling radius describes effective motion while the tire turns under load.
Why does tire deflection matter?
Deflection reduces the usable radius because the tire sidewall compresses under vehicle weight. More load usually means more compression.
Can tread wear change rolling radius?
Yes. Worn tread reduces outside diameter. This can slightly increase revolutions per mile and affect speed calculations.
What rolling factor should I use?
Use 100% for a simple estimate. Use a measured or tested factor when you have data from a tire test or vehicle setup.
How does radius affect speedometer readings?
A larger rolling radius makes actual speed higher than indicated. A smaller radius makes actual speed lower than indicated.
Can I use this for motorcycle tires?
Yes, if the tire code uses width, aspect ratio, and rim diameter. Use measured deflection for better accuracy.
Is this result exact?
No. It is an engineering estimate. Real values depend on load, pressure, rim width, temperature, tire construction, and road conditions.