GarageCalc What 175/55R15 means
175 is the section width in millimeters (6.9 in). 55 is the aspect ratio: the sidewall is 55 percent of the width, so 175 × 0.55 = 96 mm (3.8 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 × 96 = 574 mm, or 22.6 in.
A full marking adds a load index and speed rating after the size, for example 175/55R15 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 175/55R15
Going up one wheel size, the closest match is 195/45R16 at +1.5% diameter with a 88 mm sidewall instead of 96 mm. Going down one size, 165/65R14 is -0.6% with a taller 107 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 175/55R15 tire?
22.6 inches (574 mm) in overall diameter: the 15-inch wheel (381 mm) plus two 96 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 175/55R15 in inches?
22.6 x 6.9 R15: 22.6 inches tall, 6.9 inches wide, on a 15-inch wheel. The sidewall is 3.8 inches and the circumference is 70.9 inches, so the tire turns 893 times per mile.
What tire sizes are equivalent to 175/55R15?
Within 3 percent of the same diameter on the same 15-inch wheel: 195/50R15 (+0.4%), 165/60R15 (+1.0%), 155/60R15 (-1.1%); on a 16-inch wheel: 195/45R16 (+1.5%); on a 14-inch wheel: 165/65R14 (-0.6%), 185/60R14 (+0.7%). The percentage is the diameter change, which is also your speedometer error.
Can I use 185/55R15 instead of 175/55R15?
Dimensionally yes: 185/55R15 is 1.9% taller (+11 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 58.9 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 185 mm.