Tire Size Box

335/25R21 vs 315/30R21

315/30R21 compared with 335/25R21

+3.1% diameter, −0.8" width

315/30R21 is 28.44" tall vs 27.59". More than the 3% rule of thumb, so check carefully before switching.

27.6"21" rim3.3"13.2" wide
335/25R21 (solid) compared with 315/30R21 (dashed outline), drawn to scale

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

61.8 mph

315/30R21 is +0.85 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

335/25R21315/30R21Difference%
Overall diameter27.59"28.44"+0.85"+3.1%
Section width13.19"12.40"−0.79"−6.0%
Sidewall height3.30"3.72"+0.42"+12.8%
Circumference86.69"89.35"+2.66"+3.1%
Revolutions per mile730.9709.1−21.8−3.0%

Speedometer difference

If your vehicle came with 335/25R21 and you fit 315/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 335/25R21 or 315/30R21?
315/30R21 is taller by 0.85 inches (28.44" vs 27.59"). 335/25R21 is wider by 0.79 inches.
Can I replace 335/25R21 with 315/30R21?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 335/25R21), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 315/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.42 inches of ground clearance at the axle.

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