305/30 R21 Tyre Size — Meaning, Fitment & Alternatives
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What 305/30 R21 means
305/30 R21 is a 305 mm wide tyre with a sidewall 30% of that width (92 mm), on a 21-inch rim. Overall diameter works out at about 716 mm (28.2 in). It typically appears on performance cars, hot hatches and optioned-up executive models.
UK: check prices for 305/30 R21 tyres at Tirendo US: check 305/30 R21 prices at WheelHero
305/30 R21 broken down
| Code part | Value | Meaning |
|---|---|---|
| Width | 305 | Section width in millimetres, sidewall to sidewall |
| Profile (aspect ratio) | 30 | Sidewall height as a percentage of width — 92 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 | 716 mm (28.2 in) |
| Circumference | 2251 mm (88.6 in) |
| Sidewall height | 92 mm |
| Revolutions per mile | ≈ 715 |
| Revolutions per km | ≈ 444 |
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
Ultra-low-profile fitments like this trade comfort for steering response. They are more vulnerable to pothole damage, so many owners in this class carry a tyre repair kit. 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 305/30 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 716 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 |
|---|---|---|
| 235/55 R18 | 716 mm | -0.1% |
| 235/50 R19 | 718 mm | +0.2% |
| 265/30 R22 | 718 mm | +0.2% |
| 225/75 R15 | 718 mm | +0.3% |
| 265/35 R21 | 719 mm | +0.3% |
| 295/35 R20 | 714 mm | -0.3% |
| 235/45 R20 | 720 mm | +0.4% |
| 235/60 R17 | 714 mm | -0.4% |
| 265/40 R20 | 720 mm | +0.5% |
| 255/30 R22 | 712 mm | -0.6% |
Tyre pressure for 305/30 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 305/30 R21 mean?
305/30 R21 means the tyre is 305 mm wide, its sidewall is 30% of that width (92 mm), and it fits a 21-inch wheel. The R stands for radial construction.
What is the overall diameter of a 305/30 R21 tyre?
About 716 mm (28.2 inches), computed from the size code: 21-inch rim plus two sidewalls of 92 mm. Actual measurements vary slightly by brand.
Can I fit a different size instead of 305/30 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 305/30 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.