Tire Size Box

215/50R20 vs 315/30R21

315/30R21 compared with 215/50R20

−0.1% diameter, +3.9" width

315/30R21 is 28.44" tall vs 28.46". Within the 3% rule, so usually interchangeable.

28.5"20" rim4.2"8.5" wide
215/50R20 (solid) compared with 315/30R21 (dashed outline), drawn to scale

With 315/30R21 tires, when your speedometer says 60 mph you are really going

60.0 mph

315/30R21 is −0.02 inches (−0.1%) in overall diameter. That is within the 3% most shops and makers allow.

215/50R20315/30R21Difference%
Overall diameter28.46"28.44"−0.02"−0.1%
Section width8.46"12.40"+3.94"+46.5%
Sidewall height4.23"3.72"−0.51"−12.1%
Circumference89.42"89.35"−0.07"−0.1%
Revolutions per mile708.5709.1+0.6+0.1%

Speedometer difference

If your vehicle came with 215/50R20 and you fit 315/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.0 mph0.0 mph
40 mph40.0 mph0.0 mph
50 mph50.0 mph0.0 mph
60 mph60.0 mph0.0 mph
65 mph64.9 mph−0.1 mph
70 mph69.9 mph−0.1 mph
75 mph74.9 mph−0.1 mph
80 mph79.9 mph−0.1 mph

Each size on its own

Questions

Which is bigger, 215/50R20 or 315/30R21?
215/50R20 is taller by 0.02 inches (28.46" vs 28.44"). 315/30R21 is wider by 3.94 inches.
Can I replace 215/50R20 with 315/30R21?
Usually, yes. The diameter difference is −0.1%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 315/30R21 change my speedometer?
When your speedometer reads 60 mph with 315/30R21 fitted (calibrated for 215/50R20), your real speed is 60.0 mph. Your odometer will over-count miles by about 0.1%.
Will 315/30R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.01 inches of ground clearance at the axle.

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