GarageCalc What 295/40R21 means
295 is the section width in millimeters (11.6 in). 40 is the aspect ratio: the sidewall is 40 percent of the width, so 295 × 0.40 = 118 mm (4.6 in). R is radial construction, and 21 is the wheel diameter in inches (533 mm). Stack two sidewalls on the wheel and you get the overall diameter: 533 + 2 × 118 = 769 mm, or 30.3 in.
A full marking adds a load index and speed rating after the size, for example 295/40R21 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 295/40R21
Going up one wheel size, the closest match is 275/40R22 at +1.2% diameter with a 110 mm sidewall instead of 118 mm. Going down one size, 295/45R20 is +0.5% with a taller 133 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 295/40R21 tire?
30.3 inches (769 mm) in overall diameter: the 21-inch wheel (533 mm) plus two 118 mm sidewalls. Real tires land within about 1 to 2 percent of that depending on brand, tread depth and pressure.
What is 295/40R21 in inches?
30.3 x 11.6 R21: 30.3 inches tall, 11.6 inches wide, on a 21-inch wheel. The sidewall is 4.6 inches and the circumference is 95.2 inches, so the tire turns 666 times per mile.
What tire sizes are equivalent to 295/40R21?
Within 3 percent of the same diameter on the same 21-inch wheel: 285/40R21 (-1.0%), 275/45R21 (+1.5%), 275/40R21 (-2.1%); on a 22-inch wheel: 275/40R22 (+1.2%), 285/35R22 (-1.4%); on a 20-inch wheel: 295/45R20 (+0.5%), 285/45R20 (-0.6%). The percentage is the diameter change, which is also your speedometer error.
Can I use 305/35R21 instead of 295/40R21?
Dimensionally yes: 305/35R21 is 2.9% shorter (-23 mm) and 10 mm wider, and at a true 60 mph the speedometer would read 61.8 mph. The extra width needs about 0.20 in of clearance on each side of the tire and a rim width that suits 305 mm.