Tire Size Calculator
Compare any two tire sizes. Enter the width, aspect ratio, and rim diameter for your current and new tires to see diameter difference, speedometer error, and sidewall height instantly.
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Enter your values above to see the results.
Tips & Notes
- ✓Find your current tire size on the sidewall of any existing tire — it reads as three numbers like 225/55R17 (width 225mm / aspect ratio 55% / rim 17 inches).
- ✓Stay within ±3% of your original overall diameter to maintain accurate speedometer readings and avoid interference with ABS, traction control, and stability systems.
- ✓Wider tires improve grip and handling but may require wheel well clearance checks — measure the gap between the tire and wheel arch at full steering lock before ordering.
- ✓Plus-sizing (larger rim, lower-profile tire) maintains overall diameter while stiffening the sidewall for sharper steering response — but transmits more road impact to the cabin.
- ✓Always verify that the new tire's load index meets or exceeds the original — never downgrade load rating, especially on trucks or vehicles used for towing.
Common Mistakes
- ✗Confusing aspect ratio with sidewall height in mm — the aspect ratio (e.g., 55 in 225/55R17) is a percentage of width, not a direct millimeter measurement. Sidewall = 225 × 0.55 = 123.75mm.
- ✗Entering rim diameter in mm instead of inches — rim size in tire codes is always in inches. R17 means 17 inches (432mm), not 17mm.
- ✗Selecting tires more than 3% different in overall diameter from stock — this causes persistent speedometer error and can trigger false ABS/traction control interventions.
- ✗Forgetting to check physical clearance for wider tires — a 10mm width increase may cause the tire to rub against the wheel arch or suspension components at full lock.
- ✗Matching only diameter without checking load index and speed rating — a tire must meet or exceed both ratings from the original specification for safety compliance.
Tire Size Calculator Overview
Tire size codes contain four pieces of information that determine every physical dimension of the tire. Reading the code correctly is the prerequisite for any tire size comparison or fitment decision.
Tire dimensions from the size code:
Sidewall Height (mm) = Width (mm) × Aspect Ratio / 100 | Overall Diameter (in) = Rim Diameter (in) + 2 × Sidewall Height (mm) / 25.4
EX: Tire code 225/55R17 → Sidewall = 225 × 55/100 = 123.75mm → Overall diameter = 17 + (2 × 123.75/25.4) = 17 + 9.74 = 26.74 inches → Circumference = π × 26.74 = 84.0 inches → Revolutions per mile = 63,360 / 84.0 = 754 rev/mileSpeedometer error from diameter change:
Speedometer Error (%) = (New Diameter − Original Diameter) / Original Diameter × 100
EX: Switching from 225/55R17 (26.74") to 235/50R17 (26.34") → Error = (26.34 − 26.74) / 26.74 × 100 = −1.5% → At indicated 60 mph, actual speed = 59.1 mph (reads high by 0.9 mph)Common plus-sizing combinations — maintaining diameter within 2%:
| Original Size | Plus-One (+1") | Diam Change | Width Change |
|---|---|---|---|
| 205/55R16 (24.88") | 215/45R17 (24.62") | −1.0% | +10mm |
| 225/55R17 (26.74") | 235/45R18 (26.34") | −1.5% | +10mm |
| 235/60R17 (28.07") | 245/50R18 (27.68") | −1.4% | +10mm |
| 245/65R17 (29.47") | 255/55R18 (29.07") | −1.4% | +10mm |
| 265/70R17 (31.61") | 275/60R18 (31.91") | +0.9% | +10mm |
| Load Index | Max Load (lb) | Max Load (kg) | Common On |
|---|---|---|---|
| 91 | 1,356 lb | 615 kg | Compact cars |
| 95 | 1,521 lb | 690 kg | Midsize sedans |
| 100 | 1,764 lb | 800 kg | Midsize SUVs |
| 105 | 2,039 lb | 925 kg | Full-size SUVs |
| 110 | 2,337 lb | 1,060 kg | Trucks, heavy SUVs |
| 120 | 3,086 lb | 1,400 kg | Commercial/LT |
Frequently Asked Questions
A tire marked 225/55R17 breaks down as: 225 = tread width in millimeters; 55 = aspect ratio (sidewall height is 55% of 225mm = 123.75mm); R = radial construction; 17 = rim diameter in inches. The overall diameter = rim (17 inches = 431.8mm) + 2 × sidewall (2 × 123.75mm = 247.5mm) = 679.3mm = 26.75 inches. After the size code you'll find a load index number and speed rating letter — both matter for fitment compatibility.
Speedometer error is the percentage difference between what your speedometer displays and your actual speed, caused by a change in tire circumference. A larger tire travels farther per revolution, so your actual speed is higher than the displayed speed. Most mechanics and safety standards accept up to 3% diameter difference. Beyond that, your speedometer, odometer, and driver assistance systems all report inaccurate values. Example: switching from a 26-inch to a 27-inch tire (3.85% increase) means your speedometer reads 60 mph when you are actually doing 62.3 mph.
Plus-sizing means fitting a larger rim diameter with a lower-profile tire to maintain approximately the same overall diameter. Going from 205/55R16 to 225/45R17 (plus-one) keeps overall diameter nearly identical while increasing rim size by 1 inch. Benefits: stiffer sidewall improves cornering response and steering feel; wider tread can improve dry grip. Trade-offs: shorter sidewall transmits more road vibration and impact; plus-sized tires typically cost more; ride quality on rough roads decreases. Many performance car owners plus-size for handling; long-distance drivers often prefer the original profile for comfort.
Modern ABS, traction control, and stability control systems measure wheel speed using sensors calibrated to your original tire circumference. When tire size changes significantly, the system receives inaccurate speed data. A larger tire rotating at the same speed actually moves the vehicle faster than the system calculates — it may fail to intervene when it should, or intervene unnecessarily. On all-wheel-drive vehicles, tire diameter mismatches between axles can damage the center differential because the AWD system interprets the different rotational speeds as wheel slip and continuously transfers torque.
Three clearances matter: wheel well clearance (gap between tire and arch at full steering lock and full suspension compression), caliper clearance (gap between tire/wheel and brake caliper), and suspension clearance (gap from any suspension component). A general rule: up to 10mm wider than stock usually fits without modification on most vehicles; larger increases require measuring. Your tire shop can check fitment using their vehicle database, or you can find fitment guides on forums specific to your vehicle model. Always test at full steering lock — straight-ahead clearance is not sufficient.
On all-wheel-drive and four-wheel-drive vehicles, always replace all four tires at once — different diameters between axles stress the drivetrain continuously and can damage the transfer case or center differential. On front-wheel-drive or rear-wheel-drive vehicles, replacing in pairs is acceptable: replace both front or both rear together. Never mix tires with significantly different tread depths on the same axle. If replacing only two, put the new tires on the rear axle regardless of drive configuration — rear tire failure at speed causes more dangerous oversteer than front tire failure.