305/40R21 Tire Size Specifications & Compatible Vehicles
Overall Diameter
777.4 mm
Section Width
305 mm
Sidewall Height
122.0 mm
Circumference
2442.3 mm
Revs/Mile
659
Rim Diameter
21"
Inch Equivalent
30.6x12R21
Rim Width (ETRTO)
9.5" – 12.0"
Alternative Tire Sizes
These tire sizes are within 3% overall diameter of 305/40R21 and may be used as replacements:
Understanding 305/40R21
The tire size 305/40R21 means the tire has a section width of 305mm, an aspect ratio of 40% (the sidewall height is 40% of the width), and fits a 21-inch rim.
Sidewall Height = 305 x 0.4 = 122.0 mm
Overall Diameter = (122.0 x 2) + (21 x 25.4) = 777.4 mm
Circumference = 777.4 x π = 2442.3 mm
Overall Diameter = (122.0 x 2) + (21 x 25.4) = 777.4 mm
Circumference = 777.4 x π = 2442.3 mm
Frequently Asked Questions
The 305/40R21 is a standard tire size. Check your vehicle door jamb sticker or owner manual to confirm compatibility.
A 305/40R21 tire has an overall diameter of 777.4mm (30.6 inches). The sidewall height is 122.0mm, and the total circumference is 2442mm.
For 305/40R21 tires, the ETRTO recommended rim width is 10.5". The acceptable range is 9.5" minimum to 12.0" maximum. Using a rim outside this range can cause improper bead seating and unsafe handling.
Yes, as long as the replacement tire overall diameter is within 3% of the 305/40R21 777.4mm diameter. Compatible alternatives include 245/55R20, 295/50R19, 225/60R20. Use our Tire Size Calculator to compare any two sizes.