GarageCalc What 235/35R18 means
235 is the section width in millimeters (9.3 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 235 × 0.35 = 82 mm (3.2 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 × 82 = 622 mm, or 24.5 in.
A full marking adds a load index and speed rating after the size, for example 235/35R18 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 235/35R18
Going up one wheel size, the closest match is 255/30R19 at +2.2% diameter with a 77 mm sidewall instead of 82 mm. Going down one size, 235/40R17 is -0.3% with a taller 94 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 235/35R18 tire?
24.5 inches (622 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 82 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/35R18 in inches?
24.5 x 9.3 R18: 24.5 inches tall, 9.3 inches wide, on an 18-inch wheel. The sidewall is 3.2 inches and the circumference is 76.9 inches, so the tire turns 824 times per mile.
What tire sizes are equivalent to 235/35R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 225/35R18 (-1.1%), 245/35R18 (+1.1%), 215/40R18 (+1.2%); on a 19-inch wheel: 255/30R19 (+2.2%), 225/35R19 (+3.0%); on a 17-inch wheel: 235/40R17 (-0.3%), 215/45R17 (+0.6%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/35R18 instead of 235/35R18?
Dimensionally yes: 245/35R18 is 1.1% 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 245 mm.