GarageCalc What 245/35R18 means
245 is the section width in millimeters (9.6 in). 35 is the aspect ratio: the sidewall is 35 percent of the width, so 245 × 0.35 = 86 mm (3.4 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 × 86 = 629 mm, or 24.8 in.
A full marking adds a load index and speed rating after the size, for example 245/35R18 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 245/35R18
Going up one wheel size, the closest match is 255/30R19 at +1.1% diameter with a 77 mm sidewall instead of 86 mm. Going down one size, 245/40R17 is -0.1% with a taller 98 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 245/35R18 tire?
24.8 inches (629 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 86 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 245/35R18 in inches?
24.8 x 9.6 R18: 24.8 inches tall, 9.6 inches wide, on an 18-inch wheel. The sidewall is 3.4 inches and the circumference is 77.8 inches, so the tire turns 815 times per mile.
What tire sizes are equivalent to 245/35R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 235/35R18 (-1.1%), 255/35R18 (+1.1%), 225/40R18 (+1.4%); on a 19-inch wheel: 255/30R19 (+1.1%), 225/35R19 (+1.8%); on a 17-inch wheel: 245/40R17 (-0.1%), 225/45R17 (+0.9%). The percentage is the diameter change, which is also your speedometer error.
Can I use 255/35R18 instead of 245/35R18?
Dimensionally yes: 255/35R18 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 255 mm.