Tire Size Box

235/30R21 vs 235/40R21

235/40R21 compared with 235/30R21

+7.0% diameter, 0.0" width

235/40R21 is 28.40" tall vs 26.55". More than the 3% rule of thumb, so check carefully before switching.

26.6"21" rim2.8"9.3" wide
235/30R21 (solid) compared with 235/40R21 (dashed outline), drawn to scale

With 235/40R21 tires, when your speedometer says 60 mph you are really going

64.2 mph

235/40R21 is +1.85 inches (+7.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/30R21235/40R21Difference%
Overall diameter26.55"28.40"+1.85"+7.0%
Section width9.25"9.25"0.00"0.0%
Sidewall height2.78"3.70"+0.93"+33.3%
Circumference83.41"89.23"+5.81"+7.0%
Revolutions per mile759.6710.1−49.5−6.5%

Speedometer difference

If your vehicle came with 235/30R21 and you fit 235/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.1 mph+2.1 mph
40 mph42.8 mph+2.8 mph
50 mph53.5 mph+3.5 mph
60 mph64.2 mph+4.2 mph
65 mph69.5 mph+4.5 mph
70 mph74.9 mph+4.9 mph
75 mph80.2 mph+5.2 mph
80 mph85.6 mph+5.6 mph

Each size on its own

Questions

Which is bigger, 235/30R21 or 235/40R21?
235/40R21 is taller by 1.85 inches (28.40" vs 26.55"). 235/30R21 is the same width.
Can I replace 235/30R21 with 235/40R21?
It is not recommended without checking carefully. The diameter difference is +7.0%, 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 235/40R21 change my speedometer?
When your speedometer reads 60 mph with 235/40R21 fitted (calibrated for 235/30R21), your real speed is 64.2 mph. Your odometer will under-count miles by about 7.0%.
Will 235/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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