Tire Size Box

255/45R21 vs 255/55R21

255/55R21 compared with 255/45R21

+6.7% diameter, 0.0" width

255/55R21 is 32.04" tall vs 30.04". More than the 3% rule of thumb, so check carefully before switching.

30.0"21" rim4.5"10.0" wide
255/45R21 (solid) compared with 255/55R21 (dashed outline), drawn to scale

With 255/55R21 tires, when your speedometer says 60 mph you are really going

64.0 mph

255/55R21 is +2.01 inches (+6.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R21255/55R21Difference%
Overall diameter30.04"32.04"+2.01"+6.7%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.52"+1.00"+22.2%
Circumference94.36"100.67"+6.31"+6.7%
Revolutions per mile671.5629.4−42.1−6.3%

Speedometer difference

If your vehicle came with 255/45R21 and you fit 255/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.7 mph+2.7 mph
50 mph53.3 mph+3.3 mph
60 mph64.0 mph+4.0 mph
65 mph69.3 mph+4.3 mph
70 mph74.7 mph+4.7 mph
75 mph80.0 mph+5.0 mph
80 mph85.3 mph+5.3 mph

Each size on its own

Questions

Which is bigger, 255/45R21 or 255/55R21?
255/55R21 is taller by 2.01 inches (32.04" vs 30.04"). 255/45R21 is the same width.
Can I replace 255/45R21 with 255/55R21?
It is not recommended without checking carefully. The diameter difference is +6.7%, 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 255/55R21 change my speedometer?
When your speedometer reads 60 mph with 255/55R21 fitted (calibrated for 255/45R21), your real speed is 64.0 mph. Your odometer will under-count miles by about 6.7%.
Will 255/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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