Tire Size Box

235/35R21 vs 235/40R21

235/40R21 compared with 235/35R21

+3.4% diameter, 0.0" width

235/40R21 is 28.40" tall vs 27.48". More than the 3% rule of thumb, so check carefully before switching.

27.5"21" rim3.2"9.3" wide
235/35R21 (solid) compared with 235/40R21 (dashed outline), drawn to scale

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

62.0 mph

235/40R21 is +0.93 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/35R21235/40R21Difference%
Overall diameter27.48"28.40"+0.93"+3.4%
Section width9.25"9.25"0.00"0.0%
Sidewall height3.24"3.70"+0.46"+14.3%
Circumference86.32"89.23"+2.91"+3.4%
Revolutions per mile734.0710.1−23.9−3.3%

Speedometer difference

If your vehicle came with 235/35R21 and you fit 235/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 235/35R21 or 235/40R21?
235/40R21 is taller by 0.93 inches (28.40" vs 27.48"). 235/35R21 is the same width.
Can I replace 235/35R21 with 235/40R21?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 235/35R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 235/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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