GarageCalc What 235/45R20 means
235 is the section width in millimeters (9.3 in). 45 is the aspect ratio: the sidewall is 45 percent of the width, so 235 × 0.45 = 106 mm (4.2 in). R is radial construction, and 20 is the wheel diameter in inches (508 mm). Stack two sidewalls on the wheel and you get the overall diameter: 508 + 2 × 106 = 720 mm, or 28.3 in.
A full marking adds a load index and speed rating after the size, for example 235/45R20 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 235/45R20
Going up one wheel size, the closest match is 255/35R21 at -1.1% diameter with a 89 mm sidewall instead of 106 mm. Going down one size, 235/50R19 is -0.3% with a taller 118 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 235/45R20 tire?
28.3 inches (720 mm) in overall diameter: the 20-inch wheel (508 mm) plus two 106 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 235/45R20 in inches?
28.3 x 9.3 R20: 28.3 inches tall, 9.3 inches wide, on a 20-inch wheel. The sidewall is 4.2 inches and the circumference is 89.0 inches, so the tire turns 712 times per mile.
What tire sizes are equivalent to 235/45R20?
Within 3 percent of the same diameter on the same 20-inch wheel: 255/40R20 (-1.0%), 245/45R20 (+1.3%), 245/40R20 (-2.2%); on a 21-inch wheel: 255/35R21 (-1.1%), 245/40R21 (+1.4%); on a 19-inch wheel: 235/50R19 (-0.3%), 255/45R19 (-1.0%). The percentage is the diameter change, which is also your speedometer error.
Can I use 245/45R20 instead of 235/45R20?
Dimensionally yes: 245/45R20 is 1.3% taller (+9 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.