Tire Size Box

225/30R20 vs 225/25R21

225/25R21 compared with 225/30R20

+0.5% diameter, 0.0" width

225/25R21 is 25.43" tall vs 25.31". Within the 3% rule, so usually interchangeable.

25.3"20" rim2.7"8.9" wide
225/30R20 (solid) compared with 225/25R21 (dashed outline), drawn to scale

With 225/25R21 tires, when your speedometer says 60 mph you are really going

60.3 mph

225/25R21 is +0.11 inches (+0.5%) in overall diameter. That is within the 3% most shops and makers allow.

225/30R20225/25R21Difference%
Overall diameter25.31"25.43"+0.11"+0.5%
Section width8.86"8.86"0.00"0.0%
Sidewall height2.66"2.21"−0.44"−16.7%
Circumference79.53"79.89"+0.36"+0.5%
Revolutions per mile796.7793.1−3.6−0.4%

Speedometer difference

If your vehicle came with 225/30R20 and you fit 225/25R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.1 mph+0.1 mph
30 mph30.1 mph+0.1 mph
40 mph40.2 mph+0.2 mph
50 mph50.2 mph+0.2 mph
60 mph60.3 mph+0.3 mph
65 mph65.3 mph+0.3 mph
70 mph70.3 mph+0.3 mph
75 mph75.3 mph+0.3 mph
80 mph80.4 mph+0.4 mph

Each size on its own

Questions

Which is bigger, 225/30R20 or 225/25R21?
225/25R21 is taller by 0.11 inches (25.43" vs 25.31"). 225/30R20 is the same width.
Can I replace 225/30R20 with 225/25R21?
Usually, yes. The diameter difference is +0.5%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 225/25R21 change my speedometer?
When your speedometer reads 60 mph with 225/25R21 fitted (calibrated for 225/30R20), your real speed is 60.3 mph. Your odometer will under-count miles by about 0.5%.
Will 225/25R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.06 inches of ground clearance at the axle.

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