Tire Size Box

315/25R21 vs 315/30R21

315/30R21 compared with 315/25R21

+4.6% diameter, 0.0" width

315/30R21 is 28.44" tall vs 27.20". More than the 3% rule of thumb, so check carefully before switching.

27.2"21" rim3.1"12.4" wide
315/25R21 (solid) compared with 315/30R21 (dashed outline), drawn to scale

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

62.7 mph

315/30R21 is +1.24 inches (+4.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R21315/30R21Difference%
Overall diameter27.20"28.44"+1.24"+4.6%
Section width12.40"12.40"0.00"0.0%
Sidewall height3.10"3.72"+0.62"+20.0%
Circumference85.45"89.35"+3.90"+4.6%
Revolutions per mile741.5709.1−32.3−4.4%

Speedometer difference

If your vehicle came with 315/25R21 and you fit 315/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.8 mph+1.8 mph
50 mph52.3 mph+2.3 mph
60 mph62.7 mph+2.7 mph
65 mph68.0 mph+3.0 mph
70 mph73.2 mph+3.2 mph
75 mph78.4 mph+3.4 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 315/25R21 or 315/30R21?
315/30R21 is taller by 1.24 inches (28.44" vs 27.20"). 315/25R21 is the same width.
Can I replace 315/25R21 with 315/30R21?
It is not recommended without checking carefully. The diameter difference is +4.6%, more than the 3% most shops and makers allow. It will throw off the speedometer and may affect ABS, stability control and clearance.
How much will 315/30R21 change my speedometer?
When your speedometer reads 60 mph with 315/30R21 fitted (calibrated for 315/25R21), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.6%.
Will 315/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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