How to Read Tire Size Numbers
The metric tire sizing system, standardized by the ISO 4000 specification, packs five pieces of information into a compact alphanumeric code. Understanding this notation is essential before using any tire size calculator.
Breaking Down '225/65R17 102H'
| Marking | Meaning | Value |
|---|---|---|
| 225 | Section width (mm) | 8.86 inches wide |
| 65 | Aspect ratio (%) | Sidewall = 65% of 225mm = 146mm |
| R | Construction | Radial (vs B = bias-ply) |
| 17 | Wheel diameter (inches) | Fits 17-inch rims |
| 102 | Load index | 1,874 lbs max per tire |
| H | Speed rating | 130 mph max sustained |
Calculating Overall Tire Diameter
The overall diameter formula combines wheel diameter with two sidewall heights: Overall Diameter = (2 × Sidewall Height) + Wheel Diameter. For a 225/65R17: Sidewall = 225mm × 0.65 = 146.25mm = 5.76 inches. Overall diameter = (2 × 5.76) + 17 = 28.52 inches. This number determines speedometer accuracy, ground clearance, and fender clearance.
Speed Ratings and Load Indexes
The Tire and Rim Association (TRA) and ETRTO standardize speed ratings from L (75 mph) to Y (186 mph). The NHTSA requires that replacement tires meet or exceed the speed rating specified by the vehicle manufacturer. Common ratings: S (112 mph, family sedans), H (130 mph, sport sedans), V (149 mph), W (168 mph, performance), Y (186 mph, supercars).
How Tire Size Changes Affect Your Speedometer
Your vehicle's speedometer calculates speed based on wheel rotations per minute, calibrated for the factory tire size. The National Highway Traffic Safety Administration (NHTSA) Federal Motor Vehicle Safety Standard (FMVSS) No. 127 allows speedometers to read within +4 mph or +3.5% at true 50 mph.
The Speedometer Error Formula
Speedometer Reading = True Speed × (Old Tire Diameter ÷ New Tire Diameter). If you change from 225/65R17 (28.52") to 265/70R17 (31.61"): at a true 60 mph, your speedometer reads 60 × (28.52 ÷ 31.61) = 54.1 mph — you're actually going 6 mph faster than displayed. Over a 10% diameter increase, this error is significant for both safety and speeding tickets.
Acceptable Size Change Range
Most tire professionals and vehicle manufacturers recommend staying within ±3% of the original tire diameter. SAE International (formerly Society of Automotive Engineers) guidelines suggest that changes beyond 3% require speedometer recalibration. Within the 3% range, the speedometer error stays under 2 mph at highway speeds — generally acceptable for daily driving.
| Original Tire | New Tire | Diameter Change | At True 60 mph Shows | Acceptable? |
|---|---|---|---|---|
| 225/65R17 (28.52") | 235/65R17 (29.03") | +1.8% | 58.9 mph | ✅ Yes |
| 225/65R17 (28.52") | 225/70R17 (29.24") | +2.5% | 58.5 mph | ✅ Yes |
| 225/65R17 (28.52") | 265/70R17 (31.61") | +10.8% | 54.1 mph | ❌ No |
Plus-Sizing and Minus-Sizing: Changing Wheel and Tire Combinations
Plus-sizing (larger wheels with lower-profile tires) and minus-sizing (smaller wheels with taller tires) are common modifications. The key rule: maintain the same overall diameter to preserve speedometer accuracy, ABS calibration, and odometer readings.
Plus-Size Conversion Examples
| Stock Size | Plus-1 Size | Plus-2 Size |
|---|---|---|
| 225/65R17 (28.52") | 245/55R18 (28.59") | 265/45R19 (28.44") |
| 205/55R16 (24.88") | 225/45R17 (25.02") | 245/35R18 (24.75") |
| 265/70R17 (31.61") | 275/60R18 (31.00") | 285/50R20 (31.28") |
Effects of Plus-Sizing
The Tire and Rim Association's 2024 Year Book documents the tradeoffs: Benefits: improved cornering grip (shorter sidewall = less flex), larger brake clearance, sportier appearance. Drawbacks: harsher ride quality, increased risk of pothole damage, higher cost per tire ($30–$80 more), and slightly reduced fuel economy (wider contact patch = more rolling resistance). Consumer Reports testing found that plus-2 sizing typically reduces fuel economy by 1–3% and increases road noise by 2–4 dB.
Lifted Trucks and Off-Road Tire Sizing
Off-road enthusiasts commonly upsize to 31–37" tires, well beyond the 3% guideline. The SEMA (Specialty Equipment Market Association) recommends re-gearing the differential when increasing tire diameter by more than 10% — typically from a 3.73 to 4.10 or 4.56 gear ratio — to restore factory-equivalent acceleration and transmission shift points. Many states require a valid inspection after tire modifications exceeding 2 inches in diameter.
Common Tire Sizes by Vehicle Type
Passenger Cars
| Vehicle Type | Common Sizes | Diameter Range |
|---|---|---|
| Compact (Civic, Corolla) | 205/55R16, 215/55R17 | 24.9" – 26.3" |
| Mid-size (Camry, Accord) | 215/55R17, 225/45R18 | 26.3" – 25.9" |
| Full-size (Charger, Impala) | 235/55R18, 245/45R19 | 28.2" – 27.7" |
| Sports (Mustang, 370Z) | 255/40R19, 275/35R20 | 27.0" – 27.6" |
Trucks and SUVs
| Vehicle Type | Common Sizes | Diameter Range |
|---|---|---|
| Compact SUV (RAV4, CR-V) | 225/65R17, 235/60R18 | 28.5" – 29.1" |
| Mid-size truck (Tacoma, Ranger) | 265/70R16, 265/65R17 | 30.6" – 30.6" |
| Full-size truck (F-150, RAM) | 275/65R18, 275/60R20 | 32.1" – 32.4" |
| HD truck (F-250, 2500HD) | 275/70R18, LT285/60R20 | 33.2" – 33.5" |
The Tire Placard: Your Factory Size Source
Every vehicle has a tire information placard (required by FMVSS No. 110) on the driver's door jamb showing the manufacturer-recommended tire size, inflation pressure, and maximum load capacity. The National Tire Dealers Association recommends checking this placard — not the tires currently on the car — when purchasing replacements, since previous owners may have installed non-standard sizes.
Converting Tire Sizes: Metric to Inches and Flotation Sizing
Metric to Inches Conversion
To convert metric tire sizes to inches, calculate each dimension: Width: divide mm by 25.4. Sidewall: width × aspect ratio ÷ 25.4. Overall diameter: (2 × sidewall inches) + wheel diameter. Example: 285/75R16 → Width = 11.22", Sidewall = 8.42", Diameter = 16 + (2 × 8.42) = 32.83".
Flotation (Inch) Tire Sizing: LT vs Standard
Off-road and light truck tires often use flotation sizing: 33x12.50R15LT means 33" overall diameter, 12.50" wide, fits 15" wheel, Light Truck rated. This format is more intuitive for off-road sizing since the diameter is explicit. The conversion: a 285/75R16 metric is equivalent to approximately 33×11.2R16 in flotation notation.
Understanding Load Ratings: P-Metric vs LT
P-metric tires (P225/65R17) are designed for passenger vehicles with lighter load ratings. LT tires (LT265/70R17) have reinforced sidewalls for trucks carrying heavy loads or towing. The NHTSA's tire safety guide warns that substituting P-metric tires on vehicles requiring LT-rated tires can cause dangerous overloading — the load capacity difference can be 500–1,000 lbs per tire. Always match the load rating to your vehicle's GVWR (Gross Vehicle Weight Rating) on the door placard.
Step-by-Step Instructions
- 1Enter your current (original) tire size in metric format (e.g., 225/65R17).
- 2Enter the new (comparison) tire size you're considering.
- 3View the side-by-side comparison: diameter, width, sidewall height, circumference.
- 4Check the speedometer impact: how the change affects speed readings.
- 5Review the diameter percentage difference (keep within ±3% for safety).
- 6Use the visual comparison to see the relative size difference.