315/30R21
Overall diameter 722.4 mm
315/30R21 — 722.4 mm tall, and what it swaps with
315/30R21 is 315 mm wide with a 30% sidewall on a 21-inch wheel. That sidewall is 94.5 mm, so the tyre stands 722.4 mm tall and turns 440.6 times per kilometre.
- Section width
- 315 mm
- Aspect ratio
- 30 %
- Wheel size
- 21 in (533.4 mm)
- Sidewall height
- 94.5 mm
- Overall diameter
- 722.4 mm (28.44 in)
- Circumference
- 2269.5 mm
- Turns per km
- 440.6
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.
225/70R16721.4 mm · -0.14% you go slower225/65R17724.3 mm · +0.26% you go faster265/40R20720 mm · -0.33% you go slower235/45R20719.5 mm · -0.4% you go slower275/35R21725.9 mm · +0.48% you go faster265/35R21718.9 mm · -0.48% you go slower265/30R22717.8 mm · -0.64% you go slower225/60R18727.2 mm · +0.66% you go faster235/50R19717.6 mm · -0.66% you go slower245/50R19727.6 mm · +0.72% you go faster275/40R20728 mm · +0.78% you go faster305/30R21716.4 mm · -0.83% 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 315/30R21?
722.4 mm — the 533.4 mm wheel plus the 94.5 mm sidewall at top and bottom.
Q. How many turns per kilometre does 315/30R21 make?
Its circumference is 2269.5 mm, so 440.6 turns.
Q. What can I swap it for?
12 sizes stay within 3% of its diameter — the closest is 225/70R16 at 721.4 mm.
Q. Why is the sidewall 94.5 mm?
Because it is 30% of the 315 mm width. The aspect ratio is a proportion of the width, not a length.