Methodology & sources

The math

metric sidewall (mm) = width × aspect ratio ÷ 100
diameter (in) = wheel + 2 × sidewall ÷ 25.4
flotation: diameter = first number, width = second
circumference = π × diameter; revs per mile = 63,360 ÷ circumference
actual speed = indicated × new diameter ÷ old diameter

These give nominal (unloaded, calculated) figures and are labelled that way everywhere. The same code runs in your browser and in our build, and it is tested against hand-worked cases (e.g. 265/70R17 → 275/65R18 = +1.5% diameter; 60 mph indicated ≈ 60.9 mph) and against makers' published specs: our diameter must be within 2% of every published diameter in our test set.

Makers' published diameters

Size pages show the overall diameter and revolutions per mile that makers print in their data books:

• BFGoodrich passenger & light truck data book (82 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)
• Falken passenger & light truck data book (284 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)
• Goodyear passenger & light truck data book (264 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)
• Hankook passenger & light truck data book (297 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)
• Nitto product catalog (318 sizes parsed; manufacturer catalog, copy hosted by distributor U.S. AutoForce)
• Toyo Tires light truck data book (375 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)
• Uniroyal passenger & light truck data book (66 sizes parsed; manufacturer data book, copy hosted by distributor U.S. AutoForce)

A program reads each data book and only takes a diameter when it sits next to the published section width in the same row; anything else is skipped rather than guessed. Every published figure must be within 3% of the calculated diameter and makers must agree within 4% on a size; otherwise the site doesn't build until a person reviews it. Values we excluded after review:

• 225/65R17 Goodyear 27.5: Single Goodyear row 3.6% under nominal; every other maker lists 28.5-28.9 in. Treated as a data-book error.
• 235/55R19 Goodyear 28.2: One row (Kia Carnival fitment) 3.3% under nominal while other makers list 29.2-29.5 in. Likely a data-book error.
• 255/55R20 Goodyear 30.1: One row 3.0% under nominal; other makers ~31 in. Likely a data-book error.
• 255/70R17 Goodyear 29.5: One row (Ford Ranger fitment) 5% under nominal; other makers 31.0-31.2 in. Data-book error.
• 265/70R16 Goodyear 28.8: One row 5.9% under nominal (looks like another size's figures); other makers 30.5-30.7 in.
• 265/65R17 Goodyear 29.7: Same Goodyear table (p.130) as the 265/70R16 error; other makers incl. other Goodyear rows list 30.6 in.
• 235/40R19 Toyo 25.4: One Toyo row 3.8% under nominal; other makers list ~26.4-26.5 in.
• 265/70R18 Goodyear 31.7: One LT row 2.8% under nominal while other Goodyear rows for the same size list 32.6-32.9 in; its revs (638) match a 32.6 in tire, so the diameter is a misprint.

Last checked 2026-11-05. An automated job re-downloads the data books monthly and flags any change.

What we don't do

We don't say any size fits a vehicle, and we don't calculate load capacity at a given pressure. Vehicle fitment depends on wheel width and offset, clearance, load and speed ratings and maker approval; check your placard and owner's manual and ask a tire professional.

Brand names identify the source of a published figure only. TireMath isn't affiliated with any tire maker or retailer. Found an error? hello@rockypanda.com