GarageCalc What 275/70R18 means
275 is the section width in millimeters (10.8 in). 70 is the aspect ratio: the sidewall is 70 percent of the width, so 275 × 0.70 = 193 mm (7.6 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 × 193 = 842 mm, or 33.2 in.
A full marking adds a load index and speed rating after the size, for example 275/70R18 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 275/70R18
No common 19-inch size stays within 3 percent of 33.2 in. Going down one size, 255/80R17 is -0.3% with a taller 204 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.
Which cars come with 275/70R18
Typical factory fitments include Ram 1500 Rebel and Ford F-150 Tremor (2021 on). Tire size changes with trim and wheel option, so confirm on the door-jamb placard before buying.
Frequently asked questions
How tall is a 275/70R18 tire?
33.2 inches (842 mm) in overall diameter: the 18-inch wheel (457 mm) plus two 193 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 275/70R18 in inches?
33.2 x 10.8 R18: 33.2 inches tall, 10.8 inches wide, on an 18-inch wheel. The sidewall is 7.6 inches and the circumference is 104.2 inches, so the tire turns 608 times per mile.
What tire sizes are equivalent to 275/70R18?
Within 3 percent of the same diameter on the same 18-inch wheel: 265/70R18 (-1.7%), 285/70R18 (+1.7%), 285/65R18 (-1.7%); on a 17-inch wheel: 255/80R17 (-0.3%), 295/70R17 (+0.3%). The percentage is the diameter change, which is also your speedometer error.
Can I use 285/70R18 instead of 275/70R18?
Dimensionally yes: 285/70R18 is 1.7% taller (+14 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 59.0 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 285 mm.