Tire size converter: inch and metric

Two ways of writing a size

Nitto, a tire maker, explains the inch size with the example 35x12.50R17:

“The first number is the overall diameter of the tire in inches when inflated to maximum pressure. The second number is the width of the tire’s tread, in inches. The third number, again preceded by the letter “R” for radial, is the diameter of the wheel, same as the metric sizing.” [Nitto]

NHTSA explains the metric size:

“in passenger car tire sizes (e.g., 185/65R14), the first three numbers indicate the nominal width of the tire, i.e., the width in millimeters from sidewall edge to sidewall edge (185).” [Tire Fuel Efficiency Consumer Information Program (final rule)]

“The second two numbers in the size designation indicate the ratio of tire height to tire width, or the aspect ratio (65).” [Tire Fuel Efficiency Consumer Information Program (final rule)]

“The last two numbers in the size designation indicate the rim diameter code (14), or the wheel or rim diameter in inches. A change in any of these three numbers indicates a different size specification for a replacement tire.” [Tire Fuel Efficiency Consumer Information Program (final rule)]

How the converter works

Metric to inches. The sidewall height is the width times the aspect ratio: 285 × 70 / 100 = 199.5 mm, or 7.85 inches (25.4 mm to the inch). The overall diameter is the rim plus two sidewalls: 17 + 2 × 7.85 = 32.71 inches, and the width is 285 / 25.4 = 11.22 inches. So 285/70R17 is about 32.7x11.2R17 written the inch way.

Inches to metric. There is no exact match, so the converter tries every metric width that ends in 5 within 20 mm of the inch width, and every aspect ratio from 25 to 85 in steps of 5, on the same rim. It keeps the sizes whose nominal diameter is within 3 percent of the inch size's and lists the closest first. For 33x12.50R17 the width is 12.50 × 25.4 = 317.5 mm and the target diameter 33 inches:

• 315/65R17: 33.12 in tall (+0.37%), 12.40 in wide
• 325/65R17: 33.63 in tall (+1.92%), 12.80 in wide
• 305/65R17: 32.61 in tall (-1.18%), 12.01 in wide

Why 3 percent: it is the guideline BFGoodrich gives for any change of size.

“As a general guideline, keep overall diameter within about 3 percent of original. Beyond that, speedometer and odometer error becomes significant, ABS and stability control calibration is affected, and on some vehicles transmission shift points are calculated from wheel speed.” [BFGoodrich]

Arithmetic, not a catalog. A size that works out on paper is not always one a tire maker builds. Before you look for a used tire in one of these metric sizes, check that a tire maker lists it, and read its measured diameter there.

Example inch sizes and their nearest metric sizes

Examples only (not a list of the most common sizes); nominal figures from arithmetic, closest first. Check that a maker lists a size before you shop for it.

Before you buy the other size

• These are nominal figures from the numbers in the size. Nitto measures an inch size's diameter at maximum pressure [Nitto], and real tires of one size differ by model; the tire maker's data sheet gives the measured size.
• Whether a metric size is actually made, and in which load range, depends on the maker. A tire marked LT is a light-truck tire: compare the load index and load range with your vehicle's, not just the diameter.
• A size that differs from your vehicle's also changes the speedometer reading: the size comparison shows by how much.