Tire Size Box

355/40R21 vs 355/45R21

355/45R21 compared with 355/40R21

+4.3% diameter, 0.0" width

355/45R21 is 33.58" tall vs 32.18". More than the 3% rule of thumb, so check carefully before switching.

32.2"21" rim5.6"14.0" wide
355/40R21 (solid) compared with 355/45R21 (dashed outline), drawn to scale

With 355/45R21 tires, when your speedometer says 60 mph you are really going

62.6 mph

355/45R21 is +1.40 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

355/40R21355/45R21Difference%
Overall diameter32.18"33.58"+1.40"+4.3%
Section width13.98"13.98"0.00"0.0%
Sidewall height5.59"6.29"+0.70"+12.5%
Circumference101.10"105.49"+4.39"+4.3%
Revolutions per mile626.7600.6−26.1−4.2%

Speedometer difference

If your vehicle came with 355/40R21 and you fit 355/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 355/40R21 or 355/45R21?
355/45R21 is taller by 1.40 inches (33.58" vs 32.18"). 355/40R21 is the same width.
Can I replace 355/40R21 with 355/45R21?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 355/45R21 change my speedometer?
When your speedometer reads 60 mph with 355/45R21 fitted (calibrated for 355/40R21), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 355/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.70 inches of ground clearance at the axle.

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