285/40R21
Overall diameter 761.4 mm
285/40R21 — 761.4 mm tall, and what it swaps with
285/40R21 is 285 mm wide with a 40% sidewall on a 21-inch wheel. That sidewall is 114 mm, so the tyre stands 761.4 mm tall and turns 418.1 times per kilometre.
- Section width
- 285 mm
- Aspect ratio
- 40 %
- Wheel size
- 21 in (533.4 mm)
- Sidewall height
- 114 mm
- Overall diameter
- 761.4 mm (29.98 in)
- Circumference
- 2392 mm
- Turns per km
- 418.1
Sizes you can swap to
Only sizes whose overall diameter stays within 3% are listed. A different diameter covers a different distance per turn, and that gap becomes speedometer error.
235/65R18762.7 mm · +0.17% you go faster255/50R20763 mm · +0.21% you go faster255/55R19763.1 mm · +0.22% you go faster255/60R18763.2 mm · +0.24% you go faster255/65R17763.3 mm · +0.25% you go faster285/35R22758.3 mm · -0.41% you go slower295/35R22765.3 mm · +0.51% you go faster235/55R20766.5 mm · +0.67% you go faster275/45R20755.5 mm · -0.77% you go slower245/45R21753.9 mm · -0.99% you go slower275/40R21753.4 mm · -1.05% you go slower245/50R20753 mm · -1.1% you go slower
Same profile, different width
Other sizes for this wheel
21-inch wheels
245/35R21 704.9mm245/40R21 729.4mm245/45R21 753.9mm255/35R21 711.9mm255/40R21 737.4mm265/35R21 718.9mm265/40R21 745.4mm265/45R21 771.9mm275/30R21 698.4mm275/35R21 725.9mm275/40R21 753.4mm275/45R21 780.9mm285/30R21 704.4mm285/35R21 732.9mm285/40R21 761.4mm295/35R21 739.9mm305/30R21 716.4mm315/30R21 722.4mm
How the code reads
The 205 is millimetres, the 55 is a percentage of that width, and only the 16 is inches. The R stands for radial construction.
How to read this
- The first three digits are the width in mm, the middle two are the aspect ratio in percent, the last two the wheel in inches.
- Sidewall height = width × aspect ÷ 100. For 205/55 that is 112.75 mm.
- Overall diameter = wheel diameter + twice the sidewall. A 16-inch wheel is 406.4 mm.
- Once the diameter shifts by more than 3%, the speedometer and odometer drift by the same amount.
Frequently asked questions
Q. What is the overall diameter of 285/40R21?
761.4 mm — the 533.4 mm wheel plus the 114 mm sidewall at top and bottom.
Q. How many turns per kilometre does 285/40R21 make?
Its circumference is 2392 mm, so 418.1 turns.
Q. What can I swap it for?
12 sizes stay within 3% of its diameter — the closest is 235/65R18 at 762.7 mm.
Q. Why is the sidewall 114 mm?
Because it is 40% of the 285 mm width. The aspect ratio is a proportion of the width, not a length.