Rolling Radius Calculator
Free static, static loaded & dynamic rolling radius plus circumference and revs per mile/km.
QUICK EXAMPLE
A 225/45R17 tire has a geometric (unloaded) radius of about 12.5 inches based on its P-metric dimensions. Under normal vehicle load, the static loaded radius is typically 3–5% smaller (around 12.0 inches), and the dynamic rolling radius while driving is slightly smaller again (about 11.8–11.9 inches). That corresponds to roughly 800–820 revolutions per mile, which is the key input for accurate speedometer calibration, gear ratio planning, and ABS/ESP tuning.
Quick select:
Dynamic Rolling Radius
11.929in
Effective radius while rolling under load — best value to use for speedometer and gearing calculations.
Free Radius
12.486 in
Unloaded, from size markings
Static Loaded Radius
12.049 in
Under vehicle weight, stationary
Dynamic Rolling Radius
11.929 in
While rolling under load
Overall Diameter (Free)
24.97 in
Dynamic Diameter
23.86 in
Circumference
74.95 in
Based on dynamic diameter
Revs / Mile & / km
845 rpm
525.3 rev/km
How It's Calculated
Section width = 225 ÷ 25.4 = 8.858 in
Sidewall height = 8.858 × 0.45 = 3.986 in (101.3 mm)
Free radius = (rim 17 in + 2 × sidewall) ÷ 2 = 12.486 in
Static loaded radius ≈ free radius × 0.965 = 12.049 in
Dynamic radius ≈ static radius × 0.99 = 11.929 in
Revs/mile = 63,360 ÷ circumference = 845 rpm
Actual rolling radius depends on tire brand and model, load, inflation pressure, speed, and temperature. These factors (≈3–5% static deflection and ≈1–2% extra for dynamic) are typical for passenger vehicles and are suitable for speedometer and gearing estimates. For critical calibration, measure rolling circumference on the vehicle.
AI Insight
Powered by AIGet a plain-English explanation of your results — what they mean for your vehicle and driving experience.
Rolling Radius Reference Table (Approximate)
| Tire Size | Unloaded Radius (in) | Static Loaded (in) | Dynamic Radius (in) | Revs / Mile | Circumference (in) |
|---|---|---|---|---|---|
| 195/50R15 | 11.3 | 10.8 | 10.7 | 930 | 67.9 |
| 225/45R17 | 12.5 | 12.0 | 11.9 | 810 | 74.0 |
| 245/40R18 | 12.8 | 12.3 | 12.1 | 790 | 76.0 |
| 275/40R19 | 13.8 | 13.2 | 13.0 | 735 | 81.7 |
| 305/35R20 | 14.1 | 13.5 | 13.3 | 720 | 83.5 |
Values are theoretical, based on standard tire dimension formulas and typical deflection factors for passenger vehicles. Always refer to manufacturer data and real‑world measurements for final calibration.
How to Use This Calculator
- Enter your tire size in P‑metric format (e.g., 225/45R17) or use the width/aspect/rim fields.
- The calculator computes free (unloaded) radius, static loaded radius, and dynamic rolling radius using typical deflection factors.
- Review dynamic circumference and calculated revolutions per mile (and per kilometer).
- Use the results to estimate speedometer error, compare gearing changes, or evaluate suspension and ride‑height effects.
Formula & Explanation
About Rolling Radius & Why It Matters
What Is Rolling Radius?
Rolling radius (often called effective rolling radius) is the distance from the wheel center to the ground that best represents how far the vehicle travels per wheel revolution under real operating conditions. It differs from the tire’s geometric radius because a loaded, rolling tire flattens in the contact patch, so the center of the wheel is closer to the ground than the free static radius would suggest.
Free, Static Loaded & Dynamic Radius
- Free (unloaded) radius: Half the overall diameter of an inflated tire with no load applied. This is determined directly from the size markings.
- Static loaded radius: Radius from wheel center to ground when the tire is loaded but not moving. Typically 3–5% smaller than the free radius for passenger tires.
- Dynamic (effective) rolling radius: Radius the tire behaves as while rolling, effectively equal to rolling circumference divided by 2π. It is usually slightly smaller than static loaded radius and is the best choice for speed, gearing, and rev‑per‑mile calculations.
How Rolling Radius Affects Your Car
- Speedometer & odometer: Changing rolling radius changes how far you travel per wheel revolution. Larger rolling radius makes the speedometer tend to read lower than actual speed; smaller radius makes it read higher.
- Gearing & RPM: Taller effective tires reduce engine RPM at a given road speed, while shorter ones raise RPM, affecting fuel economy, acceleration, and cruising comfort.
- ABS, traction control & AWD: Modern systems expect certain wheel speeds at a given vehicle speed. Large changes in rolling radius front‑to‑rear or compared to OEM can confuse these systems or stress AWD drivetrains.
- Ride height & suspension: Larger rolling radius raises the car and can change suspension geometry and bump‑stop clearance; smaller radius does the opposite.