245/40 R21 Tyre Size — Meaning, Fitment & Alternatives
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What 245/40 R21 means
245/40 R21 is a 245 mm wide tyre with a sidewall 40% of that width (98 mm), on a 21-inch rim. Overall diameter works out at about 729 mm (28.7 in). It typically fits sports trims of hatchbacks and saloons, and performance-badged compacts.
UK: check prices for 245/40 R21 tyres at Tirendo US: check 245/40 R21 prices at WheelHero
245/40 R21 broken down
| Code part | Value | Meaning |
|---|---|---|
| Width | 245 | Section width in millimetres, sidewall to sidewall |
| Profile (aspect ratio) | 40 | Sidewall height as a percentage of width — 98 mm here |
| R | Radial | Radial construction (standard on modern car tyres) |
| Rim | 21 | Wheel diameter in inches (533 mm) |
Computed dimensions
| Measure | Value |
|---|---|
| Overall diameter | 729 mm (28.7 in) |
| Circumference | 2291 mm (90.2 in) |
| Sidewall height | 98 mm |
| Revolutions per mile | ≈ 702 |
| Revolutions per km | ≈ 436 |
These are computed reference values from the size code arithmetic — actual measurements vary slightly by brand and model. Working in different units? Convert mm to inches with the unit converters at 247QuickTools.
Tyre Owner Essentials
Typical vehicles
Lower-profile fitments in this class usually carry higher speed ratings (V, W or Y). Replacements should match the vehicle's specified rating. To confirm the exact original-equipment sizes for your car, look up your car's full specs, including OE tyre sizes, at 247VehicleCheck.
Checking your current tread? Owners often keep a tread depth gauge in the glovebox — UK legal minimum is 1.6 mm across the central three-quarters of the tread.
Speed and load ratings commonly sold in 245/40 R21
Tyres in this size are usually sold in higher speed ratings — V (149 mph), W (168 mph) and Y (186 mph) — with load indices matched to performance and premium models. See the full speed rating table and load index table for what each code is rated to.
Alternative & equivalent sizes (±3% diameter)
Sizes within ±3% of this tyre's 729 mm overall diameter keep your speedometer and gearing close to accurate. Always check clearance, rim width compatibility and load rating before switching, and never mix constructions across an axle.
| Alternative size | Overall diameter | Difference |
|---|---|---|
| 225/55 R19 | 730 mm | +0.1% |
| 245/45 R20 | 728 mm | -0.1% |
| 275/45 R19 | 730 mm | +0.1% |
| 285/30 R22 | 730 mm | +0.1% |
| 315/35 R20 | 728 mm | -0.1% |
| 245/50 R19 | 728 mm | -0.2% |
| 275/40 R20 | 728 mm | -0.2% |
| 225/60 R18 | 727 mm | -0.3% |
| 275/35 R21 | 726 mm | -0.5% |
| 285/35 R21 | 733 mm | +0.5% |
Tyre pressure for 245/40 R21
Correct pressure depends on your vehicle and load, not on the tyre size alone — always follow the placard inside the driver's door or the handbook. Typical car pressures fall between 30 and 36 psi. For psi-to-bar maths, the calculators at 247Calculators can help. A monthly check with an accurate gauge catches slow leaks before they wear the shoulders.
Recommended Accessories
FAQ
What does 245/40 R21 mean?
245/40 R21 means the tyre is 245 mm wide, its sidewall is 40% of that width (98 mm), and it fits a 21-inch wheel. The R stands for radial construction.
What is the overall diameter of a 245/40 R21 tyre?
About 729 mm (28.7 inches), computed from the size code: 21-inch rim plus two sidewalls of 98 mm. Actual measurements vary slightly by brand.
Can I fit a different size instead of 245/40 R21?
Only if the alternative stays within about ±3%% of the original overall diameter, fits your rim width, clears the bodywork and meets the placard's load and speed ratings. When in doubt, keep the placard size.
What pressure should 245/40 R21 tyres run at?
Pressure is set by the vehicle, not the tyre size — check the placard inside the driver's door or your handbook. Typical car values fall between 30 and 36 psi.
Related pages on this site
Last updated:
Data: computed reference values (ETRTO tyre-size arithmetic), build date 22 July 2026. Reference only — always follow your vehicle manufacturer's placard or handbook.