285/40R21 tire size explained
Every digit in 285/40R21 decoded: section width, aspect ratio, rim diameter, and the derived overall diameter and revolutions per mile that your speedometer and TPMS calibrations depend on.
The size string, decoded
(mm) / 40aspect ratio
(% of width) R 21rim diameter
(inches)
Computed dimensions
| Section width | 285 mm (11.22 in) |
|---|---|
| Aspect ratio | 40% (sidewall = 40% of width) |
| Rim diameter | 21 in (533.4 mm) |
| Sidewall height | 114.0 mm (4.49 in) |
| Overall diameter | 761.4 mm (29.98 in) |
| Circumference | 2392 mm (94.17 in) |
| Revolutions per mile | 673 revs/mi |
Overall diameter is the single most important number for fitment compatibility, because it determines how far the tire travels per revolution. Speedometer, odometer, TPMS rev/mile calibration, ABS rotational reference, and AWD viscous coupling all consume overall diameter (directly or indirectly). Per ETRTO 2024 §2.3, the formula is:
overall_diameter_mm = rim_inches × 25.4 + 2 × (width_mm × aspect_percent / 100)
For 285/40R21, this resolves to 21 × 25.4 + 2 × (285 × 40 / 100) = 533.4 + 228.0 = 761.4 mm.
Try changing the numbers
The calculator below mirrors the same formula your speedometer and ABS modules use. Edit any field to see how the dimensions move.
For full safe-alternative analysis use the tire compatibility calculator. To explore upsize and downsize options for 285/40R21, see 285/40R21 alternatives.
Where 285/40R21 shows up
285/40R21 appears as an OEM or approved fitment on 30 vehicle/year combinations in our catalog. The earliest model year in our data is 2016 on the Audi Q7. The size is most heavily used by Audi, which is consistent with the segment positioning the dimensions imply.
This is the same data the size's reverse-lookup page renders in table form. The explained page tells you what the size means; the reverse-lookup page tells you what cars use it.
What changes if any of the numbers move
Width up 10 mm (e.g. 285 → 295): contact patch widens, dry grip and steering load both increase, fuel economy drops slightly. Overall diameter rises 8.0 mm.
Aspect down 5 points (e.g. 40 → 35): sidewall shortens by 14.3 mm, steering sharpens, ride harshness rises, pothole risk increases. Overall diameter drops by twice that.
Rim up 1 inch (e.g. 21 → 22): rim diameter rises 25.4 mm. To preserve overall diameter (the standard "plus-1" pattern), drop aspect by ~10 points and add ~10 mm of width simultaneously. See the plus-size calculator for the math.
What the R means: radial construction
The R between aspect and rim diameter indicates radial construction: the tire's internal cords run perpendicular to the direction of travel, from bead to bead. The alternative is bias-ply construction (denoted by D, B, or no letter at all), where cords run diagonally and stack on each other. Radial tires dominate the passenger-tire market because they give a longer-lived contact patch, lower rolling resistance, and better high-speed stability. Bias-ply survives in specialty applications: trailer tires (where the stiffer sidewall is an advantage), some agricultural and classic-vehicle restorations, and occasional motorcycle fitments. The R designation has been standard on US-market passenger tires since the late 1970s and is mandatory on every modern car tire under FMVSS 109.
You'll occasionally see ZR in older size strings (e.g. 225/45ZR17). The Z signifies the tire is rated above 240 km/h (149 mph) — historically the highest speed rating before V, W, and Y were added to the ladder. On current tires the speed rating is encoded separately after the load index, but the ZR notation persists on legacy and high-performance models.
What's NOT in the size string
The width-aspect-rim trio defines geometry only. Several other critical specifications appear elsewhere on the sidewall and matter just as much for fitment compatibility:
- Load index — a 2- or 3-digit number after the size (e.g. 225/65R17 102 H) that maps to a maximum-weight rating per Tire & Rim Association 2025 Table 1-2. An OEM replacement must equal or exceed the original load index.
- Speed rating — a single letter after the load index (e.g. 285/40R21 102 H) on the ETRTO ladder (S=180, T=190, U=200, H=210, V=240, W=270, Y=300 km/h). Replacement should match or exceed the placard speed rating.
- OEM approval markings — small letters next to the size: T0 (Tesla), MO and MO-S (Mercedes), * (BMW), N0/N1/N2/N3/N4 (Porsche), F (Ford performance), AO (Audi), VOL (Volvo), J (Jaguar). These signal the tire was developed against that manufacturer's specifications.
- HL (high load) — appears before the width on many EV tires (e.g. HL235/45R20). Indicates the tire carries a higher load index than the standard XL designation, required for the additional mass of battery-electric vehicles.
- DOT date code — a 4-digit code at the end of the DOT string (e.g. "DOT XYZ1 ABC2 4724") indicates the week and year of manufacture (here, the 47th week of 2024). Tires older than 6 years from the date code carry sidewall degradation risk regardless of tread depth.
The tire sidewall codes guide walks through every marking position on a typical sidewall.
Sources & methodology
Last verified 2026-05-17 against the standards below.
- ETRTO 2024 Standards Manual §2.3 (section width, aspect ratio, overall diameter formula). European Tyre and Rim Technical Organisation, Brussels.
- Tire & Rim Association 2025 Yearbook (North American passenger tire dimensions and load index reference). T&RA, Copley OH.
- ISO 4000-1:2021 Passenger car tyres and rims — Part 1: Tyre designations. International Organization for Standardization.
- SAE J1349 (speedometer accuracy tolerance).