Tire Size Calculator

Compare tire sizes and speedometer error

This tire size calculator compares two metric or flotation tire codes and reports nominal diameter, section width, sidewall height, circumference, revolutions per mile and kilometer, speedometer error, odometer change, and static ground-clearance change. It can also search for metric sizes near the original rolling diameter when changing wheel diameter.

Enter the size printed on the tire sidewall, such as 225/65R17, LT285/70R17, or 33x12.50R15. Prefixes such as P, LT, ST, and T are accepted for geometry calculations, but the prefix, load range, service description, and construction still matter when choosing a real replacement.

How to read a metric tire size

A metric code such as 225/65R17 describes a 225 mm nominal section width, a sidewall height equal to 65% of that width, radial construction, and a 17-inch wheel. The wheel diameter is converted to millimeters before adding both sidewalls.

sidewall height = section width x aspect ratio / 100

overall diameter = wheel diameter + 2 x sidewall height

circumference = pi x overall diameter

The calculated width and diameter are nominal. A tire manufacturer's specification sheet may list a different measured width or diameter because the result depends on the measuring rim, product design, inflation pressure, load, and tread condition.

Flotation tire sizes

An inch-designated flotation size such as 33x12.50R15 gives nominal overall diameter first, nominal section width second, and wheel diameter last. The calculator accepts the stated diameter and width directly, then derives an approximate sidewall height and aspect ratio for comparison. It does not convert the size into an official equivalent metric designation.

Speedometer and odometer changes

A larger tire travels farther per wheel revolution. If the vehicle remains calibrated for the original tire, actual road speed and distance change in the same proportion as tire circumference. Choose mph and miles or km/h and kilometers; the ratio is unit-independent.

actual speed = indicated speed x new diameter / original diameter

actual distance = indicated distance x new diameter / original diameter

The speed table shows several common readings so the effect is easy to scan. A positive difference means the vehicle is moving farther or faster than indicated. A smaller tire produces the opposite effect.

Ride height, width, and revolutions

Half of the diameter change is the approximate static change at the axle. A tire that is 1 inch larger in diameter raises the axle about 0.5 inch before changes caused by load, pressure, tread, or suspension geometry. Section-width change is shown separately because a similar diameter does not guarantee adequate fender or suspension clearance.

ground-clearance change = (new diameter - original diameter) / 2

revolutions per mile = 63,360 / circumference in inches

Calculated revolutions use an unloaded geometric circumference. For calibration work, use the tire manufacturer's published revolutions-per-mile value when available because rolling circumference under load can differ.

Find similar-diameter tire sizes

Choose “Find similar-diameter sizes,” enter a target wheel diameter, and select a nominal diameter tolerance. The calculator evaluates common metric width and aspect-ratio increments and ranks the closest mathematical matches. These are search candidates, not purchase recommendations. A generated size may be unavailable or unsuitable for the wheel and vehicle.

Changing to a larger wheel while keeping a similar total tire diameter is often called plus sizing. The tire sidewall normally becomes shorter as wheel diameter increases. Similar outside diameter can reduce speedometer change, but it does not verify ride quality, load capacity, brake clearance, steering clearance, or compatibility with all-wheel-drive systems.

Tire-size comparison example

Changing from 225/65R17 to 235/60R18 increases calculated diameter from about 28.52 to 29.10 inches, a difference of about 2.06%. At an indicated 60 mph, calculated road speed is about 61.23 mph, and static ground clearance rises about 0.29 inch. The new tire is also about 10 mm wider by nominal section width.

What a diameter tolerance means

The result labels changes within 1% as a very close nominal diameter, within 2% as close, within 3% as noticeable, and larger changes as substantial. These labels describe only mathematical diameter difference. Three percent is often used as a screening rule by comparison tools, but it is not a universal safety limit or a substitute for manufacturer approval.

Vehicle systems, effective gearing, wheel width, load capacity, speed rating, clearances, tire construction, and manufacturer requirements all matter. On four-wheel-drive or all-wheel-drive vehicles, differences between tires on the same vehicle may also be tightly limited by the vehicle manufacturer.

Verify a replacement tire

NHTSA TireWise advises checking the owner's manual or Tire and Loading Information Label on the driver's side door edge or post for the correct tire size and cold inflation pressure. NHTSA also recommends consulting a tire dealer when there is doubt about the correct size.

Before buying, confirm the exact tire's load index or load range, speed rating, wheel-width range, published dimensions, cold inflation requirements, and clearance at full steering lock and suspension compression. The pressure molded on the tire sidewall is not the vehicle manufacturer's recommended operating pressure.

Frequently asked questions

Why can the listed diameter differ from this result?

The formula uses nominal size-code dimensions. Consult the tire manufacturer's specification sheet for measured overall diameter and revolutions per mile.

Does this calculator confirm wheel fitment?

No. It compares tire geometry only. Bolt pattern, wheel offset, hub bore, brake clearance, fender clearance, suspension clearance, wheel-width compatibility, and load ratings require separate verification.

Can I compare mph and km/h?

Yes. Select the speed and distance unit. Diameter ratio is the same in either system, so only the displayed speed and distance labels change.

Does a larger tire make the speedometer read low?

With calibration unchanged, a larger tire generally makes actual speed higher than the indicated speed because each revolution covers more distance.

Are the similar-size results guaranteed to be manufactured?

No. The list contains nominal metric geometry candidates. Confirm that a size exists and is approved for the vehicle and wheel before use.