Tire Static Loaded Radius Calculator
Calculate the static loaded radius (SLR) — the distance from your wheel axle center to the ground — for any tire size, corner load, and inflation pressure. Also outputs dynamic rolling radius, contact patch size, tire stiffness rate, and revolutions per km/mile. Uses ETRTO / ISO 4000-1 and SAE J2530 formulas.
Quick SLR Reference
| Scenario | Instant Reference Value |
|---|---|
| 205/55R16 under standard load | Static loaded radius: 295 mm (unloaded: 306 mm) |
| 265/70R17 under standard load | Static loaded radius: 372 mm (unloaded: 388 mm) |
| 295/75R22.5 (commercial truck) | Static loaded radius: 510 mm (unloaded: 535 mm) |
| General deflection rate | Loaded radius ≈ unloaded radius × 0.96–0.97 |
SLR is measured at rated load + inflation, at rest. Used for ABS calibration, speedometer calibration, and towing calculations.
Quick Presets
Tire & Vehicle Inputs
Static Loaded Radius (SLR)
322.11mm
12.682 in · Deflection 3.3 mm (3.0% SH)
Sidewall Height
109.5mm
H = W × AR / 100
Free Outer Diameter
650.8mm
Unloaded, no deflection
Free (Unloaded) Radius
325.4mm
12.811 in
Deflection
3.3mm
3.0% of sidewall height
Static Loaded Radius
322.1mm
12.682 in
Ground Clearance Ref
322.1mm
Axle center to ground
Get a plain-English explanation of your results — what they mean for your vehicle and driving experience.
How It Was Calculated — Step by Step
SLR Reference Table — Common Tire Sizes
| Tire Size | SLR (mm) | SLR (in) | Deflection % | Contact Patch | Vehicle Type |
|---|---|---|---|---|---|
| 195/50R15 | 272 | 10.7 | 4.8% | 145×84 mm | Economy |
| 205/55R16 | 294 | 11.6 | 4.2% | 155×89 mm | Touring |
| 225/45R17 | 303 | 11.9 | 3.5% | 165×82 mm | Sport |
| 235/55R18 | 336 | 13.2 | 4.0% | 175×94 mm | CUV/SUV |
| 255/50R19 | 348 | 13.7 | 3.8% | 183×98 mm | SUV/EV |
| 265/70R17 | 378 | 14.9 | 5.2% | 196×110 mm | Truck/4WD |
| 275/40R20 | 347 | 13.7 | 3.2% | 188×86 mm | Performance |
| 305/35R22 | 374 | 14.7 | 3.0% | 204×88 mm | Ultra-low |
Values at typical corner loads (360–480 kg) and recommended pressures per ETRTO / manufacturer data.
Frequently Asked Questions
Static Loaded Radius Reference Table — 8 Common Tire Sizes
Pre-calculated SLR values at typical corner loads (360–480 kg) and manufacturer-recommended pressures. Use these as a sanity check against the calculator output for your tire.
| Tire Size | Free Radius | Static Loaded Radius | Deflection | Contact Patch | Vehicle Type |
|---|---|---|---|---|---|
| 195/50R15 | 276.5 mm | 272 mm / 10.7 in | 4.5 mm (4.8%) | ~145 × 84 mm | Economy / compact |
| 205/55R16 | 298.7 mm | 294 mm / 11.6 in | 4.7 mm (4.2%) | ~155 × 89 mm | Touring / passenger |
| 225/45R17 | 307.3 mm | 303 mm / 11.9 in | 4.3 mm (3.5%) | ~165 × 82 mm | Sport / performance |
| 235/55R18 | 340.5 mm | 336 mm / 13.2 in | 4.5 mm (3.4%) | ~175 × 94 mm | CUV / crossover |
| 255/50R19 | 352.7 mm | 348 mm / 13.7 in | 4.7 mm (3.7%) | ~183 × 98 mm | SUV / EV |
| 265/70R17 | 384.6 mm | 378 mm / 14.9 in | 6.6 mm (5.2%) | ~196 × 110 mm | Truck / 4WD |
| 275/40R20 | 351.0 mm | 347 mm / 13.7 in | 4.0 mm (3.2%) | ~188 × 86 mm | Performance / sports car |
| 305/35R22 | 377.9 mm | 374 mm / 14.7 in | 3.9 mm (3.0%) | ~204 × 88 mm | Ultra-low profile / luxury |
Values at typical OEM corner loads and recommended cold pressures per ETRTO / manufacturer data. Actual SLR varies with vehicle weight distribution, load index, rim width, and temperature.
How to Calculate Tire Static Loaded Radius
Follow these seven steps to get accurate SLR, DRR, contact patch, and stiffness outputs matched to your exact tire, vehicle load, and operating conditions.
- 1
Enter or select a tire size preset
Type your tire's section width (mm), aspect ratio (%), and rim diameter (inches) — or click one of the 8 preset buttons (e.g. 205/55R16, 265/70R17) to populate all fields instantly. These numbers appear on the tire sidewall in the format WIDTH/ASPECTRRIM.
- 2
Select the load index
Choose the load index from your tire's sidewall (e.g. 91, 99, 107). The dropdown shows the corresponding maximum load in kg and lbs per ETRTO standards. The load index is used to calculate tire stiffness rate and validate that your corner load is within the tire's rated capacity.
- 3
Enter corner load (kg)
Input the weight on this corner of the vehicle in kg. A rough estimate: divide curb weight by 4, then add 10–20 kg for the front axle bias on a typical FWD vehicle. For example, a 1,520 kg car with 58% front bias has approximately 380 kg + 50 kg = 430 kg front-corner load. Corner load is the single largest factor in SLR calculation.
- 4
Enter inflation pressure (psi)
Enter the cold inflation pressure in PSI from your vehicle's door-jamb placard or owner's manual (not the max pressure on the tire sidewall). Typical range: 28–45 psi. Higher pressure raises tire stiffness, reduces deflection, and increases SLR. The calculator shows the sensitivity of SLR to ±5 psi in the Stiffness tab.
- 5
Enter vehicle speed (km/h)
Enter your operating speed in km/h for the dynamic rolling radius (DRR) correction. At 0 km/h, DRR equals SLR. The ETRTO standard test is 60 km/h. At highway speeds (110–130 km/h), centrifugal force slightly reduces sidewall deflection, increasing DRR by 1–3 mm above SLR.
- 6
(Optional) Enable rim width correction
If your actual rim width differs from the tire's design-spec rim, click 'Rim Width ETRTO Correction' and enter the actual rim width in inches. The calculator applies the ETRTO ±5 mm section width / ±2.5 mm sidewall height correction per 0.5 inch rim width deviation.
- 7
Read results across four tabs
The Geometry tab shows free radius, SLR, and deflection. The Dynamic tab shows DRR, circumference, and rev/km. The Contact tab shows contact patch dimensions and a live visual footprint. The Stiffness tab shows tire spring rate and sensitivity panels for pressure and corner load. Use 'Copy Results' for a formatted export.
SLR vs Free Radius vs Dynamic Rolling Radius — What's the Difference?
Free (Unloaded) Radius
R_free = (D_rim + 2H) / 2
The geometric radius of the tire in the air with no load. Equals half the outer diameter. Never achieved in real driving — the tire is always compressed under vehicle weight.
Static Loaded Radius
SLR = R_free − δ
The axle-to-ground distance with the vehicle stationary under its actual weight. The primary reference dimension for suspension geometry, ride height, and arch clearance. Published in all manufacturer tire dimension tables.
Dynamic Rolling Radius
DRR = SLR × (1 + 0.0003V)
The effective radius while the tire rolls at speed. Centrifugal force partially restores sidewall deformation, so DRR > SLR. Used for speedometer and ABS calibration. ETRTO test speed: 60 km/h.
Why Static Loaded Radius Matters — Engineering Context
SLR is the master reference dimension for chassis and suspension engineers. Every ride height calculation, suspension K&C (kinematics and compliance) setup, and wheel-arch clearance analysis starts from SLR. When you change tire size — going from a 225/45R17 to a 235/40R18 for example — the SLR difference directly determines whether the new tire will rub the arch at full jounce, whether the speedometer will read high or low, and whether the front-to-rear ride height balance is maintained. A 5 mm SLR increase raises the entire body by 5 mm relative to the road.
Speedometer error is directly proportional to DRR deviation from OEM. Vehicle ECUs calculate road speed by counting wheel-speed sensor pulses per unit time and dividing by the OEM-calibrated rolling circumference (2π × DRR_OEM). If you fit a tire with a DRR 2% larger than OEM, your speedometer will read 2% low — showing 98 km/h when you are actually doing 100 km/h. Most countries legally allow a speedometer reading 0–10% high (over-reading). An under-reading speedometer is technically illegal in many jurisdictions. This calculator outputs the exact revolutions-per-km difference, which you can use to calculate the speedometer error percentage for any tire change.
AWD and 4WD systems are acutely sensitive to SLR mismatches. In a full-time AWD system, the center differential allows a small speed difference between front and rear axles. If the front tires have a significantly different SLR from the rear (due to size changes, wear differences, or inflation variation), the differential operates in a constant partial-lock state, generating heat and wear. Most AWD manufacturers specify a maximum of 5 mm circumference difference (≈ 0.8 mm radius difference) between all four tires. Exceeding this — for example by fitting a spare tire with a different size — can damage the transfer case in less than 100 km of driving.
Tire deflection is not a flaw — it is designed energy storage. The 10–18% sidewall deflection in a normal passenger car tire is intentional. It acts as a secondary spring in the suspension system, absorbing road imperfections before they reach the spring and damper. This is why reducing tire pressure increases ride comfort but reduces handling precision — more deflection means a softer, less predictable contact patch. Race cars on dry tracks use near-maximum recommended pressure to minimise deflection and maximise contact patch stiffness and consistency.
Deflection Percentage Guide by Tire Category
Normal operating deflection range as a percentage of sidewall height, at OEM pressures and typical corner loads.
| Tire Category | Aspect Ratio Range | Normal Deflection % | Stiffness (N/mm) | Notes |
|---|---|---|---|---|
| Ultra-low profile (sport) | 25–35 | 8–12% | 250–350 | Hard ride, precise handling |
| Low profile (performance) | 35–45 | 10–14% | 180–260 | Sport / EV typical |
| Standard passenger car | 50–60 | 13–17% | 130–200 | Most common range |
| Touring / high-profile | 65–75 | 15–20% | 100–160 | Comfort-focused |
| LT / truck (LR-E rated) | 70–85 | 14–19% | 220–400 | Higher load index, stiffer carcass |
| Run-flat (ZP) | 45–55 | 4–8% | 350–600 | Reinforced sidewall — stiff ride |
| Agricultural / specialty | 70–100+ | 18–28% | 60–120 | Large deflection by design |