Tire Size Box

225/40R21 vs 225/45R21

225/45R21 compared with 225/40R21

+3.2% diameter, 0.0" width

225/45R21 is 28.97" tall vs 28.09". More than the 3% rule of thumb, so check carefully before switching.

28.1"21" rim3.5"8.9" wide
225/40R21 (solid) compared with 225/45R21 (dashed outline), drawn to scale

With 225/45R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

225/45R21 is +0.89 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/40R21225/45R21Difference%
Overall diameter28.09"28.97"+0.89"+3.2%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.54"3.99"+0.44"+12.5%
Circumference88.24"91.02"+2.78"+3.2%
Revolutions per mile718.1696.1−22.0−3.1%

Speedometer difference

If your vehicle came with 225/40R21 and you fit 225/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 225/40R21 or 225/45R21?
225/45R21 is taller by 0.89 inches (28.97" vs 28.09"). 225/40R21 is the same width.
Can I replace 225/40R21 with 225/45R21?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 225/45R21 change my speedometer?
When your speedometer reads 60 mph with 225/45R21 fitted (calibrated for 225/40R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 225/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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