Tire Size Box

285/40R21 vs 285/45R21

285/45R21 compared with 285/40R21

+3.7% diameter, 0.0" width

285/45R21 is 31.10" tall vs 29.98". More than the 3% rule of thumb, so check carefully before switching.

30.0"21" rim4.5"11.2" wide
285/40R21 (solid) compared with 285/45R21 (dashed outline), drawn to scale

With 285/45R21 tires, when your speedometer says 60 mph you are really going

62.2 mph

285/45R21 is +1.12 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/40R21285/45R21Difference%
Overall diameter29.98"31.10"+1.12"+3.7%
Section width11.22"11.22"0.00"0.0%
Sidewall height4.49"5.05"+0.56"+12.5%
Circumference94.17"97.70"+3.53"+3.7%
Revolutions per mile672.8648.5−24.3−3.6%

Speedometer difference

If your vehicle came with 285/40R21 and you fit 285/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 285/40R21 or 285/45R21?
285/45R21 is taller by 1.12 inches (31.10" vs 29.98"). 285/40R21 is the same width.
Can I replace 285/40R21 with 285/45R21?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 285/45R21 change my speedometer?
When your speedometer reads 60 mph with 285/45R21 fitted (calibrated for 285/40R21), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 285/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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