GarageCalc What 185/55R15 means
185 is the section width in millimeters (7.3 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 185 × 0.55 = 102 mm (4.0 in). R is radial construction, and 15 is the wheel diameter in inches (381 mm). Stack two sidewalls on the wheel and you get the overall diameter: 381 + 2 × 102 = 585 mm, or 23.0 in.
A full marking adds a load index and speed rating after the size, for example 185/55R15 91V. Those two codes set how much weight and speed the tire is certified for; look them up in the load index chart and the speed rating chart.
Plus and minus sizing from 185/55R15
Going up one wheel size, the closest match is 195/45R16 at -0.4% diameter with a 88 mm sidewall instead of 102 mm. Going down one size, 175/65R14 is -0.2% with a taller 114 mm sidewall for more comfort and pothole margin. Anything inside 3 percent keeps the speedometer, ABS and gearing effectively stock; the comparison calculator shows the full speedometer table for any pair.
Frequently asked questions
How tall is a 185/55R15 tire?
23.0 inches (585 mm) in overall diameter: the 15-inch wheel (381 mm) plus two 102 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 185/55R15 in inches?
23.0 x 7.3 R15: 23.0 inches tall, 7.3 inches wide, on a 15-inch wheel. The sidewall is 4.0 inches and the circumference is 72.3 inches, so the tire turns 876 times per mile.
What tire sizes are equivalent to 185/55R15?
Within 3 percent of the same diameter on the same 15-inch wheel: 205/50R15 (+0.3%), 165/60R15 (-0.9%), 175/60R15 (+1.1%); on a 16-inch wheel: 195/45R16 (-0.4%), 205/45R16 (+1.1%); on a 14-inch wheel: 175/65R14 (-0.2%), 165/70R14 (+0.4%). The percentage is the diameter change, which is also your speedometer error.
Can I use 195/50R15 instead of 185/55R15?
Dimensionally yes: 195/50R15 is 1.5% shorter (-9 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 60.9 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 195 mm.