Tire Size Calculator

How to use this tire size calculator

Start with the tire size currently on your vehicle, then enter the tire size you are considering. The calculator updates as soon as you type, so you can compare several options without pressing a calculate button. If you have a size printed on a sidewall, paste it into either tire-size field. Formats such as 205/55R16, 205/55 R16, P205/55R16, and LT285/70R17 are accepted. Common flotation sizes such as 31x10.50R15 are converted into the closest metric-style inputs for comparison.

The default display is US-first: inches, mph, and revolutions per mile. Use the metric toggle when you want millimeters and revolutions per kilometer. The share button copies a URL with the current sizes in the query string, which is useful when you want to save a comparison or send it to a tire shop. The verdict card is a quick guide, but the table is where you should review the actual numbers before making a decision.

What tire size numbers mean

A metric tire size such as 265/70R17 has three main numbers. The first number, 265, is the section width in millimeters. This is the approximate width of the tire from sidewall to sidewall, not the tread width. The second number, 70, is the aspect ratio. It means the sidewall height is 70% of the tire width. The R means radial construction, which is standard for modern passenger and light-truck tires. The final number, 17, is the wheel rim diameter in inches.

Because tire sizing mixes metric and inch units, diameter is not obvious from the sidewall code. A 265/70R17 tire has a sidewall height of 185.5 mm, and the 17-inch wheel adds 431.8 mm. Add two sidewalls plus the wheel diameter and the overall diameter is about 802.8 mm, or 31.61 inches. That diameter drives most of the differences you see in the calculator.

How tire diameter affects speedometer accuracy

Your vehicle estimates road speed by counting rotations through the drivetrain and wheel-speed sensors. If the new tire is taller than the original tire, each rotation covers more distance. That means the vehicle is moving faster than the speedometer shows. If the new tire is shorter, each rotation covers less distance, so the vehicle is moving slower than indicated.

For example, changing from a 30-inch tire to a 31.5-inch tire is a 5% diameter increase. At an indicated 60 mph, actual speed is about 63 mph. That may affect speed limits, odometer readings, shift behavior, driver-assistance calibration, and fuel-economy estimates. Trucks and SUVs often tolerate modest changes, but newer vehicles with advanced stability control can be more sensitive than older models.

What tire size difference is safe?

A common rule of thumb is to keep overall tire diameter within 3% of the original size. Within that range, the speedometer error is usually modest and fitment is more likely to be manageable. A 3% to 5% change is not automatically unsafe, but it deserves a more careful check. Look at fender clearance, suspension clearance, brake clearance, wheel width, wheel offset, load rating, and whether the vehicle has been lifted or lowered.

Changes beyond 5% should not be treated as a simple tire swap. The vehicle may need recalibration, different wheels, trimming, lift components, gearing changes, or professional inspection. On all-wheel-drive vehicles, mixing tire diameters can also create drivetrain stress, so compare all four tires and follow the vehicle manufacturer guidance.

Tire clearance and fitment notes

Ground clearance changes by half of the diameter difference. If the new tire is one inch taller overall, the axle and body gain about half an inch of ground clearance. That sounds simple, but fitment is three-dimensional. Diameter can cause rubbing at the front or rear of the wheel well while turning. Width can cause rubbing on the inner liner, suspension, sway bar, or fender lip. Wheel offset can move the tire inward or outward and change both inner and outer clearance.

Tire models also vary. Two tires with the same printed size can measure differently when mounted and inflated. Tread design, wheel width, load range, and pressure all affect real-world measurements. Use this calculator as a first filter, then verify the exact tire specs from the manufacturer and inspect the vehicle at full steering lock and suspension compression.

Tire size formulas used

The calculator uses standard tire geometry. Rim diameter in millimeters equals rim inches multiplied by 25.4. Sidewall height equals section width multiplied by the aspect ratio divided by 100. Overall diameter equals rim diameter plus two sidewalls. Circumference equals pi multiplied by overall diameter. Revolutions per mile equals 1,609,344 divided by circumference in millimeters, and revolutions per kilometer equals 1,000,000 divided by circumference in millimeters.

Speedometer impact is based on the ratio between new diameter and old diameter. Actual speed equals indicated speed multiplied by that ratio. The calculator reports the error at 30, 60, and 70 mph because those speeds cover common city, highway, and interstate checks in the US. Percentage differences are calculated as the new value minus the old value, divided by the old value, multiplied by 100.