How to read a tire size
A metric tire size such as 265/70R17 packs three measurements into one code:
- 265 is the section width in millimeters, measured from sidewall to sidewall at the widest point.
- 70 is the aspect ratio: the sidewall height as a percentage of the width. Here the sidewall is 70% of 265 mm.
- R means radial construction. You may also see ZR on high-speed tires; it does not change the dimensions.
- 17 is the wheel (rim) diameter in inches.
A P in front (P265/70R17) marks a passenger tire, and LT marks a light-truck tire. These prefixes change the load rating and construction, not the basic size math. The numbers after the size, such as 113T, are the load index and speed rating.
Off-road and truck tires often use flotation (inch) sizes, such as 33x12.50R15. These list the overall diameter (33 in), the section width (12.50 in) and the rim diameter (15 in) directly.
The formulas
Because the width is in millimeters and the rim is in inches, the sidewall has to be converted with 25.4 mm per inch before the two are added.
Worked example: 265/70R17
Sidewall = 265 × 70 ÷ 100 = 185.5 mm, or 7.30 in.
Diameter = 2 × 7.30 + 17 = 31.61 in (802.8 mm).
Circumference = π × 31.61 ≈ 99.3 in, so the tire turns about 638 times per mile. The section width is 265 ÷ 25.4 ≈ 10.4 in, so the inch equivalent is roughly 31.6x10.4R17.
Why calculated sizes differ from the real tire
The size code describes the nominal size. The tire you buy can differ for several reasons:
- Brand and tread design: aggressive mud-terrain tires are often a little taller than highway tires in the same size. Many "33-inch" flotation tires actually measure 32.5 to 33 in.
- Rim width: section width is specified on a particular measuring rim. A wider or narrower wheel changes the width slightly.
- Load and inflation: a loaded tire squats, so its effective rolling radius is smaller than half of the unloaded diameter. That is why the revolutions per mile in a manufacturer's spec sheet are usually a few percent higher than the geometric figure here.
- Wear: a worn tire can be around half an inch shorter in diameter than a new one, depending on tread depth.
For exact numbers, check the manufacturer's published specifications for the specific tire model.
What to do with the numbers
Diameter affects your speedometer, gearing, ground clearance and whether the tire fits in the wheel well. Width affects clearance at full steering lock and whether a tire suits your wheel. To weigh up a new size against your current one, use the tire size comparison calculator. To see how far off your speedometer would read, use the speedometer calculator. For a quick reference of popular sizes, see the tire size chart.
Frequently asked questions
How tall is a 265/70R17 tire?
About 31.6 inches (803 mm) in overall diameter, with a 7.3 inch sidewall and a 10.4 inch section width.
What does the R in a tire size mean?
R stands for radial construction, which almost all modern tires use. It is not the radius. The number after it is the wheel diameter in inches.
Does the P or LT prefix change the tire size?
No, the dimensions are calculated the same way. P (passenger) and LT (light truck) tires differ in construction and how load capacity is rated, so do not swap one for the other without checking load ratings.
Is a 285/70R17 a 33-inch tire?
It calculates to about 32.7 inches, so it is commonly called a 33-inch tire, although it is slightly shorter than a true 33-inch flotation size.
Why do tire specs list different revolutions per mile?
Manufacturers measure revolutions per mile with the tire loaded on a vehicle, which makes the effective rolling diameter smaller than the unloaded diameter. Calculated values are usually a few percent lower than published ones.
Last reviewed November 2026. Spotted an error? Tell us.