255/40R18
Overall diameter 661.2 mm
255/40R18 — 661.2 mm tall, and what it swaps with
255/40R18 is 255 mm wide with a 40% sidewall on a 18-inch wheel. That sidewall is 102 mm, so the tyre stands 661.2 mm tall and turns 481.4 times per kilometre.
- Section width
- 255 mm
- Aspect ratio
- 40 %
- Wheel size
- 18 in (457.2 mm)
- Sidewall height
- 102 mm
- Overall diameter
- 661.2 mm (26.03 in)
- Circumference
- 2077.2 mm
- Turns per km
- 481.4
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.
255/35R19661.1 mm · -0.02% you go slower255/30R20661 mm · -0.03% you go slower215/65R15660.5 mm · -0.11% you go slower225/40R19662.6 mm · +0.21% you go faster225/45R18659.7 mm · -0.23% you go slower295/30R19659.6 mm · -0.24% you go slower215/60R16664.4 mm · +0.48% you go faster205/55R17657.3 mm · -0.59% you go slower225/50R17656.8 mm · -0.67% you go slower265/30R20667 mm · +0.88% you go faster245/30R20655 mm · -0.94% you go slower205/70R15668 mm · +1.03% you go faster
Same profile, different width
Other sizes for this wheel
18-inch wheels
215/40R18 629.2mm215/45R18 650.7mm215/50R18 672.2mm225/35R18 614.7mm225/40R18 637.2mm225/45R18 659.7mm225/50R18 682.2mm225/55R18 704.7mm225/60R18 727.2mm235/40R18 645.2mm235/45R18 668.7mm235/50R18 692.2mm235/55R18 715.7mm235/60R18 739.2mm235/65R18 762.7mm245/40R18 653.2mm245/45R18 677.7mm245/50R18 702.2mm245/60R18 751.2mm255/35R18 635.7mm255/40R18 661.2mm255/45R18 686.7mm255/55R18 737.7mm255/60R18 763.2mm265/60R18 775.2mm275/35R18 649.7mm275/40R18 677.2mm275/65R18 814.7mm
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 255/40R18?
661.2 mm — the 457.2 mm wheel plus the 102 mm sidewall at top and bottom.
Q. How many turns per kilometre does 255/40R18 make?
Its circumference is 2077.2 mm, so 481.4 turns.
Q. What can I swap it for?
12 sizes stay within 3% of its diameter — the closest is 255/35R19 at 661.1 mm.
Q. Why is the sidewall 102 mm?
Because it is 40% of the 255 mm width. The aspect ratio is a proportion of the width, not a length.