245/40R21
Overall diameter 729.4 mm
245/40R21 — 729.4 mm tall, and what it swaps with
245/40R21 is 245 mm wide with a 40% sidewall on a 21-inch wheel. That sidewall is 98 mm, so the tyre stands 729.4 mm tall and turns 436.4 times per kilometre.
- Section width
- 245 mm
- Aspect ratio
- 40 %
- Wheel size
- 21 in (533.4 mm)
- Sidewall height
- 98 mm
- Overall diameter
- 729.4 mm (28.72 in)
- Circumference
- 2291.5 mm
- Turns per km
- 436.4
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.
225/55R19730.1 mm · +0.1% you go faster275/45R19730.1 mm · +0.1% you go faster245/45R20728.5 mm · -0.12% you go slower315/35R20728.5 mm · -0.12% you go slower275/40R20728 mm · -0.19% you go slower245/50R19727.6 mm · -0.25% you go slower225/60R18727.2 mm · -0.3% you go slower285/35R21732.9 mm · +0.48% you go faster275/35R21725.9 mm · -0.48% you go slower235/75R15733.5 mm · +0.56% you go faster225/65R17724.3 mm · -0.7% you go slower235/70R16735.4 mm · +0.82% you go faster
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 245/40R21?
729.4 mm — the 533.4 mm wheel plus the 98 mm sidewall at top and bottom.
Q. How many turns per kilometre does 245/40R21 make?
Its circumference is 2291.5 mm, so 436.4 turns.
Q. What can I swap it for?
12 sizes stay within 3% of its diameter — the closest is 225/55R19 at 730.1 mm.
Q. Why is the sidewall 98 mm?
Because it is 40% of the 245 mm width. The aspect ratio is a proportion of the width, not a length.