GarageCalc What 215/35R18 means
215 is the section width in millimeters (8.5 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 215 × 0.35 = 75 mm (3.0 in). R is radial construction, and 18 is the wheel diameter in inches (457 mm). Stack two sidewalls on the wheel and you get the overall diameter: 457 + 2 × 75 = 608 mm, or 23.9 in.
A full marking adds a load index and speed rating after the size, for example 215/35R18 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 215/35R18
No common 19-inch size stays within 3 percent of 23.9 in. Going down one size, 215/40R17 is -0.6% with a taller 86 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 215/35R18 tire?
23.9 inches (608 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 75 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 215/35R18 in inches?
23.9 x 8.5 R18: 23.9 inches tall, 8.5 inches wide, on an 18-inch wheel. The sidewall is 3.0 inches and the circumference is 75.2 inches, so the tire turns 843 times per mile.
What tire sizes are equivalent to 215/35R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 225/35R18 (+1.2%), 235/35R18 (+2.3%); on a 17-inch wheel: 215/40R17 (-0.6%), 225/40R17 (+0.7%). The percentage is the diameter change, which is also your speedometer error.
Can I use 225/35R18 instead of 215/35R18?
Dimensionally yes: 225/35R18 is 1.2% taller (+7 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.3 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 225 mm.