Tire Size Box

245/45R21 vs 255/45R21

255/45R21 compared with 245/45R21

+1.2% diameter, +0.4" width

255/45R21 is 30.04" tall vs 29.68". Within the 3% rule, so usually interchangeable.

29.7"21" rim4.3"9.6" wide
245/45R21 (solid) compared with 255/45R21 (dashed outline), drawn to scale

With 255/45R21 tires, when your speedometer says 60 mph you are really going

60.7 mph

255/45R21 is +0.35 inches (+1.2%) in overall diameter. That is within the 3% most shops and makers allow.

245/45R21255/45R21Difference%
Overall diameter29.68"30.04"+0.35"+1.2%
Section width9.65"10.04"+0.39"+4.1%
Sidewall height4.34"4.52"+0.18"+4.1%
Circumference93.25"94.36"+1.11"+1.2%
Revolutions per mile679.5671.5−8.0−1.2%

Speedometer difference

If your vehicle came with 245/45R21 and you fit 255/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.2 mph+0.2 mph
30 mph30.4 mph+0.4 mph
40 mph40.5 mph+0.5 mph
50 mph50.6 mph+0.6 mph
60 mph60.7 mph+0.7 mph
65 mph65.8 mph+0.8 mph
70 mph70.8 mph+0.8 mph
75 mph75.9 mph+0.9 mph
80 mph81.0 mph+1.0 mph

Each size on its own

Questions

Which is bigger, 245/45R21 or 255/45R21?
255/45R21 is taller by 0.35 inches (30.04" vs 29.68"). 255/45R21 is wider by 0.39 inches.
Can I replace 245/45R21 with 255/45R21?
Usually, yes. The diameter difference is +1.2%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 255/45R21 change my speedometer?
When your speedometer reads 60 mph with 255/45R21 fitted (calibrated for 245/45R21), your real speed is 60.7 mph. Your odometer will under-count miles by about 1.2%.
Will 255/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.18 inches of ground clearance at the axle.

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