Tire Size Box

225/35R21 vs 225/40R21

225/40R21 compared with 225/35R21

+3.3% diameter, 0.0" width

225/40R21 is 28.09" tall vs 27.20". More than the 3% rule of thumb, so check carefully before switching.

27.2"21" rim3.1"8.9" wide
225/35R21 (solid) compared with 225/40R21 (dashed outline), drawn to scale

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

62.0 mph

225/40R21 is +0.89 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/35R21225/40R21Difference%
Overall diameter27.20"28.09"+0.89"+3.3%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.10"3.54"+0.44"+14.3%
Circumference85.45"88.24"+2.78"+3.3%
Revolutions per mile741.5718.1−23.4−3.2%

Speedometer difference

If your vehicle came with 225/35R21 and you fit 225/40R21, 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 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 225/35R21 or 225/40R21?
225/40R21 is taller by 0.89 inches (28.09" vs 27.20"). 225/35R21 is the same width.
Can I replace 225/35R21 with 225/40R21?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 225/35R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 225/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

Other comparisons