315/40R21 tires

What 315/40R21 means

The first number — 315 — is the tire's section width in millimeters (about 12.4 inches from sidewall to sidewall, measured when the tire is mounted and inflated to standard pressure). The second number — 40 — is the aspect ratio: the sidewall height as a percentage of the section width, which works out to 126 mm of sidewall for this size. The R indicates radial construction (universal on passenger tires today, mandatory under FMVSS 109), and 21 is the rim diameter in inches. Together these give an overall tire diameter of 785.4 mm (30.9 inches) — the dimension that matters for speedometer accuracy, wheel-well clearance, and TPMS / ABS / AWD calibration.

100 vehicle/year combinations in our catalog list this size as an OEM or approved fitment, and 21 tire models in our catalog are sold in this size. Each one turns about 652 revolutions per mile (circumference 2467 mm × π), which is the figure your speedometer and TPMS modules are calibrated against. When you replace tires within the same size, brand and compound choice are what change the driving experience — every tire in this size is engineered to the same outside diameter, so speedometer error and wheel clearance won't change. Where the differences show up is in tread compound (longer-wearing vs stickier), construction (touring sidewall vs performance-stiff), and season class. For a deeper breakdown of what each digit in the size string represents, see the paired 315/40R21 explained page.

If you are considering deviating from 315/40R21 — a plus-size step up, a winter step down, or a same-rim width change — keep the overall outside diameter within ±3% of the original per the ETRTO 2024 §2.3 safe-fit standard. Major changes to outside diameter affect speedometer calibration (SAE J1349 ±4% factory tolerance), ABS rotational reference (FMVSS 135), TPMS rev/mile tracking (FMVSS 138), and AWD viscous coupling temperature on systems that rely on consistent tire revolutions per mile. The Compatible alternative sizes table above lists every size within tolerance, and the 315/40R21 upsize and downsize options page groups them by upgrade intent (Plus-1, Plus-2, winter narrower, wider, etc.) with verdicts and speedometer impact. Always confirm any non-OEM substitution with the manufacturer or a qualified tire shop before purchase.

For shoppers looking at this size, the key spec questions to ask are: does the tire's load index equal or exceed the OEM placard requirement (Tire & Rim Association 2025 Table 1-2 maps the number to maximum weight), does its speed rating match or exceed the placard, and what is its UTQG treadwear rating? The third question is the best single proxy for tread life: 600+ UTQG signals a long-wear touring compound, 400–600 is mid-life performance, under 300 is short-life high-grip. Cross-reference any candidate tire's spec sheet against the manufacturer's published technical bulletin before committing.