Tire Size Box

305/25R21 vs 305/30R21

305/30R21 compared with 305/25R21

+4.4% diameter, 0.0" width

305/30R21 is 28.20" tall vs 27.00". More than the 3% rule of thumb, so check carefully before switching.

27.0"21" rim3.0"12.0" wide
305/25R21 (solid) compared with 305/30R21 (dashed outline), drawn to scale

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

62.7 mph

305/30R21 is +1.20 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/25R21305/30R21Difference%
Overall diameter27.00"28.20"+1.20"+4.4%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.00"3.60"+0.60"+20.0%
Circumference84.84"88.61"+3.77"+4.4%
Revolutions per mile746.9715.1−31.8−4.3%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 305/25R21 or 305/30R21?
305/30R21 is taller by 1.20 inches (28.20" vs 27.00"). 305/25R21 is the same width.
Can I replace 305/25R21 with 305/30R21?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 305/25R21), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.4%.
Will 305/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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