Tire Size Box

275/30R20 vs 235/30R21

235/30R21 compared with 275/30R20

+0.2% diameter, −1.6" width

235/30R21 is 26.55" tall vs 26.50". Within the 3% rule, so usually interchangeable.

26.5"20" rim3.2"10.8" wide
275/30R20 (solid) compared with 235/30R21 (dashed outline), drawn to scale

With 235/30R21 tires, when your speedometer says 60 mph you are really going

60.1 mph

235/30R21 is +0.06 inches (+0.2%) in overall diameter. That is within the 3% most shops and makers allow.

275/30R20235/30R21Difference%
Overall diameter26.50"26.55"+0.06"+0.2%
Section width10.83"9.25"−1.57"−14.5%
Sidewall height3.25"2.78"−0.47"−14.5%
Circumference83.24"83.41"+0.17"+0.2%
Revolutions per mile761.2759.6−1.6−0.2%

Speedometer difference

If your vehicle came with 275/30R20 and you fit 235/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.1 mph+0.1 mph
40 mph40.1 mph+0.1 mph
50 mph50.1 mph+0.1 mph
60 mph60.1 mph+0.1 mph
65 mph65.1 mph+0.1 mph
70 mph70.1 mph+0.1 mph
75 mph75.2 mph+0.2 mph
80 mph80.2 mph+0.2 mph

Each size on its own

Questions

Which is bigger, 275/30R20 or 235/30R21?
235/30R21 is taller by 0.06 inches (26.55" vs 26.50"). 275/30R20 is wider by 1.57 inches.
Can I replace 275/30R20 with 235/30R21?
Usually, yes. The diameter difference is +0.2%, 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 235/30R21 change my speedometer?
When your speedometer reads 60 mph with 235/30R21 fitted (calibrated for 275/30R20), your real speed is 60.1 mph. Your odometer will under-count miles by about 0.2%.
Will 235/30R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.03 inches of ground clearance at the axle.

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