Tire Size Box

225/40R20 vs 255/30R21

255/30R21 compared with 225/40R20

−0.2% diameter, +1.2" width

255/30R21 is 27.02" tall vs 27.09". Within the 3% rule, so usually interchangeable.

27.1"20" rim3.5"8.9" wide
225/40R20 (solid) compared with 255/30R21 (dashed outline), drawn to scale

With 255/30R21 tires, when your speedometer says 60 mph you are really going

59.9 mph

255/30R21 is −0.06 inches (−0.2%) in overall diameter. That is within the 3% most shops and makers allow.

225/40R20255/30R21Difference%
Overall diameter27.09"27.02"−0.06"−0.2%
Section width8.86"10.04"+1.18"+13.3%
Sidewall height3.54"3.01"−0.53"−15.0%
Circumference85.10"84.90"−0.20"−0.2%
Revolutions per mile744.6746.3+1.7+0.2%

Speedometer difference

If your vehicle came with 225/40R20 and you fit 255/30R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph29.9 mph−0.1 mph
40 mph39.9 mph−0.1 mph
50 mph49.9 mph−0.1 mph
60 mph59.9 mph−0.1 mph
65 mph64.8 mph−0.2 mph
70 mph69.8 mph−0.2 mph
75 mph74.8 mph−0.2 mph
80 mph79.8 mph−0.2 mph

Each size on its own

Questions

Which is bigger, 225/40R20 or 255/30R21?
225/40R20 is taller by 0.06 inches (27.09" vs 27.02"). 255/30R21 is wider by 1.18 inches.
Can I replace 225/40R20 with 255/30R21?
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 255/30R21 change my speedometer?
When your speedometer reads 60 mph with 255/30R21 fitted (calibrated for 225/40R20), your real speed is 59.9 mph. Your odometer will over-count miles by about 0.2%.
Will 255/30R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.03 inches of ground clearance at the axle.

Other comparisons