Tire Size Box

315/25R21 vs 305/30R21

305/30R21 compared with 315/25R21

+3.7% diameter, −0.4" width

305/30R21 is 28.20" 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 305/30R21 (dashed outline), drawn to scale

With 305/30R21 tires, when your speedometer says 60 mph you are really going

62.2 mph

305/30R21 is +1.00 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R21305/30R21Difference%
Overall diameter27.20"28.20"+1.00"+3.7%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height3.10"3.60"+0.50"+16.2%
Circumference85.45"88.61"+3.15"+3.7%
Revolutions per mile741.5715.1−26.4−3.6%

Speedometer difference

If your vehicle came with 315/25R21 and you fit 305/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 315/25R21 or 305/30R21?
305/30R21 is taller by 1.00 inches (28.20" vs 27.20"). 315/25R21 is wider by 0.39 inches.
Can I replace 315/25R21 with 305/30R21?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 305/30R21 change my speedometer?
When your speedometer reads 60 mph with 305/30R21 fitted (calibrated for 315/25R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 305/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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