315/40R21 Tire Size Specifications & Compatible Vehicles
Overall Diameter
785.4 mm
Section Width
315 mm
Sidewall Height
126.0 mm
Circumference
2467.4 mm
Revs/Mile
652
Rim Diameter
21"
Inch Equivalent
30.9x12.4R21
Rim Width (ETRTO)
10.0" – 12.5"
Alternative Tire Sizes
These tire sizes are within 3% overall diameter of 315/40R21 and may be used as replacements:
Understanding 315/40R21
The tire size 315/40R21 means the tire has a section width of 315mm, an aspect ratio of 40% (the sidewall height is 40% of the width), and fits a 21-inch rim.
Sidewall Height = 315 x 0.4 = 126.0 mm
Overall Diameter = (126.0 x 2) + (21 x 25.4) = 785.4 mm
Circumference = 785.4 x π = 2467.4 mm
Overall Diameter = (126.0 x 2) + (21 x 25.4) = 785.4 mm
Circumference = 785.4 x π = 2467.4 mm
Frequently Asked Questions
The 315/40R21 is a standard tire size. Check your vehicle door jamb sticker or owner manual to confirm compatibility.
A 315/40R21 tire has an overall diameter of 785.4mm (30.9 inches). The sidewall height is 126.0mm, and the total circumference is 2467mm.
For 315/40R21 tires, the ETRTO recommended rim width is 11.0". The acceptable range is 10.0" minimum to 12.5" 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 315/40R21 785.4mm diameter. Compatible alternatives include 185/75R20, 275/55R19, 175/65R22. Use our Tire Size Calculator to compare any two sizes.