Tire Size Calculator — Diameter, Speedometer & Comparison
Enter your tire size in {width}/{aspect}R{wheel} format (e.g. 225/45R17) to get overall diameter, circumference, sidewall height, and revolutions per mile. Optionally compare two sizes for speedometer error and clearance change.
Tire Size Calculator
How to Read Tire Size
The tire sidewall lists size in the standard P-metric format: {width}/{aspect}R{wheel}. Example: 225/45R17.
- 225 — section width in millimeters (the widest point of the tire when mounted, sidewall to sidewall).
- 45 — aspect ratio, or sidewall height as a percentage of width. Lower = shorter sidewall, sportier handling.
- R — radial construction (steel belts run perpendicular to the direction of travel). 99% of modern tires are radial.
- 17 — wheel diameter in inches (the rim it mounts on).
Other markings on the sidewall include load index (e.g., 91 = 1,356 lb capacity), speed rating (e.g., V = 149 mph), DOT code (manufacture week + year), and treadwear / traction / temperature ratings (UTQG).
Common Tire Sizes
Diameter and circumference for popular fitments. Use these as a baseline when comparing alternative sizes.
| Size | Diameter | Circumference | Revs / mile | Common vehicles |
|---|---|---|---|---|
| 195/65R15 | 25.0″ | 78.6″ | 806.0 | Civic, Corolla, Mazda3 |
| 205/55R16 | 24.9″ | 78.1″ | 810.6 | Camry, Accord, Jetta |
| 225/45R17 | 25.0″ | 78.4″ | 807.7 | Sport sedans, Civic Si, GTI |
| 225/65R17 | 28.5″ | 89.6″ | 706.9 | RAV4, CR-V, Forester |
| 265/70R17 | 31.6″ | 99.3″ | 637.9 | Tacoma, Ranger, mid-size SUVs |
| 275/55R20 | 31.9″ | 100.2″ | 632.2 | Full-size SUVs, F-150 sport |
| 285/75R16 | 32.8″ | 103.1″ | 614.6 | Off-road trucks, Jeep Wrangler |
| 315/70R17 | 34.4″ | 107.9″ | 587.0 | Heavy-duty pickups, lifted trucks |
Plus Sizing Guide (+1, +2)
Plus sizing means going to a larger wheel diameter while keeping overall tire diameter the same (or within a few percent). You shorten the sidewall and widen the tread to compensate. Benefits: better steering response, more aggressive looks. Tradeoffs: harsher ride, more wheel damage risk, often higher cost.
- Stock: 225/55R17 (26.74″ diameter)
- +1: 235/45R18 (26.32″) — 1.6% smaller, acceptable
- +2: 245/40R19 (26.72″) — matches stock almost exactly
- +3: 255/35R20 (27.03″) — 1.1% larger, still within 3%
Use the calculator’s comparison fields to verify diameter delta stays within ±3% of stock.
Speedometer Accuracy & the 3% Rule
Modern car speedometers read wheel rotation rate and convert via the original tire’s circumference. If you change diameter, you change actual mph at any given indicated speed.
- Larger diameter: fewer rotations per mile → speedo reads slow (you’re actually going faster).
- Smaller diameter: more rotations per mile → speedo reads fast (you’re actually going slower).
- Within ±3%: generally acceptable; most factory speedometers have ~3% built-in tolerance and overestimate slightly for safety.
- Beyond ±3%: ABS, traction control, AWD/4WD, and odometer all become inaccurate. Some cars allow OBD-II recalibration; trucks often have dealer or aftermarket recalibration tools.
Frequently Asked Questions
What does 225/45R17 mean?
225 = tire width in millimeters. 45 = aspect ratio — sidewall height as a percentage of width (so 225 × 45% = 101.25 mm sidewall). R = radial construction (the standard today). 17 = wheel diameter in inches. Overall diameter = wheel + 2 × sidewall = 17 + 2×3.99 = 24.97 inches.
How much bigger is a 265/70R17 than a 245/75R16?
265/70R17: 17 + 2×(265 × 0.70 / 25.4) = 31.61 in. 245/75R16: 16 + 2×(245 × 0.75 / 25.4) = 30.47 in. The 265/70R17 is +1.14 in (+3.7%) taller. Speedometer reading 60 mph would actually be 62.2 mph.
Will different size tires affect my speedometer?
Yes — speedometers calibrate from wheel rotations per mile. Larger overall diameter means fewer rotations per mile, so the speedo reads slow (you’re going faster than it shows). The general rule is to stay within ±3% of original diameter to avoid noticeable error and to keep ABS, traction control, and odometer accurate.
Can I put different size tires on my car?
Within ±3% of overall diameter, generally yes — common when “plus sizing” (going to a larger wheel with a shorter sidewall to keep diameter constant). Always check load index and speed rating on the new tire matches or exceeds OEM. Beyond ±3%, you may need recalibration, encounter rubbing in turns, or affect AWD/4WD systems.
Related Tools
- Shoe Size Converter — international shoe size conversion (US, EU, UK)
- Inches to mm Converter — quick unit conversion
- mm to Inches Converter — reverse direction
- All Size Converters
Calculations use the standard P-metric tire formula: diameter = wheel + 2 × (width × aspect / 100) / 25.4. Speedometer error assumes the original tire size was used for factory calibration. Always confirm load index, speed rating, and clearance with your vehicle manufacturer or a qualified installer before changing tire sizes.
