Tire Size Box

225/30R21 vs 225/40R21

225/40R21 compared with 225/30R21

+6.7% diameter, 0.0" width

225/40R21 is 28.09" tall vs 26.31". More than the 3% rule of thumb, so check carefully before switching.

26.3"21" rim2.7"8.9" wide
225/30R21 (solid) compared with 225/40R21 (dashed outline), drawn to scale

With 225/40R21 tires, when your speedometer says 60 mph you are really going

64.0 mph

225/40R21 is +1.77 inches (+6.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/30R21225/40R21Difference%
Overall diameter26.31"28.09"+1.77"+6.7%
Section width8.86"8.86"0.00"0.0%
Sidewall height2.66"3.54"+0.89"+33.3%
Circumference82.67"88.24"+5.57"+6.7%
Revolutions per mile766.4718.1−48.3−6.3%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.7 mph+2.7 mph
50 mph53.4 mph+3.4 mph
60 mph64.0 mph+4.0 mph
65 mph69.4 mph+4.4 mph
70 mph74.7 mph+4.7 mph
75 mph80.0 mph+5.0 mph
80 mph85.4 mph+5.4 mph

Each size on its own

Questions

Which is bigger, 225/30R21 or 225/40R21?
225/40R21 is taller by 1.77 inches (28.09" vs 26.31"). 225/30R21 is the same width.
Can I replace 225/30R21 with 225/40R21?
It is not recommended without checking carefully. The diameter difference is +6.7%, 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 225/40R21 change my speedometer?
When your speedometer reads 60 mph with 225/40R21 fitted (calibrated for 225/30R21), your real speed is 64.0 mph. Your odometer will under-count miles by about 6.7%.
Will 225/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.89 inches of ground clearance at the axle.

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