Tire Size Box

235/45R21 vs 235/55R21

235/55R21 compared with 235/45R21

+6.3% diameter, 0.0" width

235/55R21 is 31.18" tall vs 29.33". More than the 3% rule of thumb, so check carefully before switching.

29.3"21" rim4.2"9.3" wide
235/45R21 (solid) compared with 235/55R21 (dashed outline), drawn to scale

With 235/55R21 tires, when your speedometer says 60 mph you are really going

63.8 mph

235/55R21 is +1.85 inches (+6.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/45R21235/55R21Difference%
Overall diameter29.33"31.18"+1.85"+6.3%
Section width9.25"9.25"0.00"0.0%
Sidewall height4.16"5.09"+0.93"+22.2%
Circumference92.13"97.95"+5.81"+6.3%
Revolutions per mile687.7646.9−40.8−5.9%

Speedometer difference

If your vehicle came with 235/45R21 and you fit 235/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph31.9 mph+1.9 mph
40 mph42.5 mph+2.5 mph
50 mph53.2 mph+3.2 mph
60 mph63.8 mph+3.8 mph
65 mph69.1 mph+4.1 mph
70 mph74.4 mph+4.4 mph
75 mph79.7 mph+4.7 mph
80 mph85.0 mph+5.0 mph

Each size on its own

Questions

Which is bigger, 235/45R21 or 235/55R21?
235/55R21 is taller by 1.85 inches (31.18" vs 29.33"). 235/45R21 is the same width.
Can I replace 235/45R21 with 235/55R21?
It is not recommended without checking carefully. The diameter difference is +6.3%, 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 235/55R21 change my speedometer?
When your speedometer reads 60 mph with 235/55R21 fitted (calibrated for 235/45R21), your real speed is 63.8 mph. Your odometer will under-count miles by about 6.3%.
Will 235/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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