Tire Size Box

245/25R21 vs 245/30R21

245/30R21 compared with 245/25R21

+3.7% diameter, 0.0" width

245/30R21 is 26.79" tall vs 25.82". More than the 3% rule of thumb, so check carefully before switching.

25.8"21" rim2.4"9.6" wide
245/25R21 (solid) compared with 245/30R21 (dashed outline), drawn to scale

With 245/30R21 tires, when your speedometer says 60 mph you are really going

62.2 mph

245/30R21 is +0.96 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/25R21245/30R21Difference%
Overall diameter25.82"26.79"+0.96"+3.7%
Section width9.65"9.65"0.00"0.0%
Sidewall height2.41"2.89"+0.48"+20.0%
Circumference81.12"84.16"+3.03"+3.7%
Revolutions per mile781.0752.9−28.1−3.6%

Speedometer difference

If your vehicle came with 245/25R21 and you fit 245/30R21, 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 mph51.9 mph+1.9 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, 245/25R21 or 245/30R21?
245/30R21 is taller by 0.96 inches (26.79" vs 25.82"). 245/25R21 is the same width.
Can I replace 245/25R21 with 245/30R21?
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 245/30R21 change my speedometer?
When your speedometer reads 60 mph with 245/30R21 fitted (calibrated for 245/25R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 245/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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