What is the difference between static and dynamic rolling radius?

Static loaded radius is the distance from the wheel center to the ground when the tire is under load but not moving. Dynamic (effective) rolling radius is the radius the tire behaves as while rolling — equal to rolling circumference divided by 2π. Dynamic radius is usually slightly smaller than static loaded radius because of how the contact patch deforms while the tire rolls.

Why does rolling radius matter for speedometer accuracy?

Speedometers assume a specific relationship between wheel revolutions and distance traveled. If you change rolling radius by fitting a taller or shorter tire, the tire covers more or less distance per revolution. A larger effective radius makes the speedometer read lower than actual speed; a smaller radius makes it read higher.

How much do load and pressure affect rolling radius?

Higher vehicle load and lower inflation pressure both increase tire deflection, reducing static loaded and dynamic rolling radius. For typical passenger tires, the reduction from unloaded to static loaded radius is often 3–5% under normal conditions, but can be more for heavily loaded or under-inflated tires.

Does changing tire size affect gearing and engine RPM?

Yes. A tire with a larger rolling radius travels farther per revolution. At a given road speed, the wheels turn more slowly, reducing engine RPM for the same gear ratio. A smaller rolling radius raises RPM at a given speed, affecting fuel economy, acceleration, and NVH.

Which radius should I use for gear ratio and speed calculations?

Use dynamic rolling radius (or the corresponding rolling circumference), because it reflects the tire under load while moving. Using only the geometric radius from size markings ignores deflection and can introduce a few percent error.

Can I use this calculator for motorcycles or bicycles?

The same geometric and rolling-circumference principles apply, but motorcycle and bicycle tires have different profiles, loads, and inflation pressures. This calculator is a useful starting point; measuring real rolling circumference on the bike is recommended for precise work.

How accurate are these rolling radius estimates?

Results are usually within a few percent of real-world values for normal passenger-vehicle loads and pressures. For critical calibration, competition vehicles, or unusual loads, measure rolling radius or rolling circumference directly on the vehicle.

Rolling Radius Calculator

Free static, static loaded & dynamic rolling radius — plus circumference and revs per mile.

Quick Answer

A 205/55R16 has a static radius of 306mm and a rolling radius of 299mm under load — about 2–3% smaller than the geometric size. That difference of ~7mm means the tire travels slightly less per revolution than the markings suggest, directly affecting speedometer accuracy and gearing calculations.

Tire	Radii
205/55R16 (common passenger car)	Static: 306mm · Rolling: 299mm (loaded)
225/45R17 (sport/performance)	Static: 312mm · Rolling: 305mm
265/70R17 (Tacoma / truck)	Static: 388mm · Rolling: 379mm
315/75R16 (off-road)	Static: 427mm · Rolling: 417mm

Rolling (dynamic) radius is typically 2–3% smaller than static radius due to tire deformation under load.

Enter P‑metric tire dimensions. Free radius is derived from size markings; static loaded and dynamic rolling radius use typical passenger-vehicle deflection factors.

Section Width (mm)

Aspect Ratio (%)

Rim Diameter (in)

Quick select:

Dynamic Rolling Radius

11.929in

Effective radius while rolling under load — use for speedometer and gearing calculations.

225/45R17

Free Radius

12.486 in

317.1 mm · unloaded

Static Loaded Radius

12.049 in

306.0 mm · under weight

Dynamic Rolling Radius

11.929 in

303.0 mm · while rolling

Overall Ø (Free)

24.97 in

Dynamic Ø

23.86 in

Circumference

74.95 in

Revs / Mile

845

525.3 rev/km

Step-by-step

Section width = 225 ÷ 25.4 = 8.858 in

Sidewall = 8.858 × 0.45 = 3.986 in (101.3 mm)

Free radius = (rim 17 in + 2 × sidewall) ÷ 2 = 12.486 in

Static loaded ≈ free × 0.965 = 12.049 in

Dynamic ≈ static × 0.99 = 11.929 in

Revs/mile = 63,360 ÷ 74.95 = 845

Deflection factors (3.5% static, 1% dynamic) are typical for passenger vehicles under normal load and pressure. For critical calibration, measure rolling circumference on the vehicle.

AI Insight

Get a plain-English explanation of your results — what they mean for your vehicle and driving experience.

Rolling Radius Reference Table

Tire Size	Free Radius (in)	Static Loaded (in)	Dynamic (in)	Revs/Mile	Circ (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

Theoretical values based on standard tire-dimension formulas and typical deflection factors. Verify with manufacturer data for critical calibration.

Formula & Deflection Factors

Free Radius Section width (in) = width (mm) ÷ 25.4 Sidewall (in) = section width × aspect ratio ÷ 100 Overall dia (in) = rim dia (in) + 2 × sidewall Free radius = overall dia ÷ 2 Static Loaded Radius Static loaded ≈ free radius × 0.965 (~3.5% deflection) Dynamic Rolling Radius Dynamic ≈ static loaded × 0.99 (~1% additional) Revolutions per Mile Circumference (in) = π × dynamic diameter Revs/mile = 63,360 ÷ circumference

Free radius — from size markings, no load
Static loaded — tire under vehicle weight, stationary
Dynamic rolling — best value for speed, gearing, and rev-per-mile calculations

How to Use This Calculator

1Enter section width: Input the tire's section width in millimeters — the first number in the P-metric size (e.g. 225 in 225/45R17).
2Enter aspect ratio: Input the aspect ratio percentage — the number after the slash (e.g. 45).
3Enter rim diameter: Input the rim diameter in inches — the number after R (e.g. 17).
4Read the three radii: The result shows free (unloaded) radius, static loaded radius, and dynamic rolling radius — each in inches and millimeters.
5Use revs per mile for calibration: The revolutions per mile figure is the key input for speedometer calibration and gearing calculations after a tire size change.

Frequently Asked Questions

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