Tire Size Box

235/45R20 vs 235/55R20

235/55R20 compared with 235/45R20

+6.5% diameter, 0.0" width

235/55R20 is 30.18" tall vs 28.33". More than the 3% rule of thumb, so check carefully before switching.

28.3"20" rim4.2"9.3" wide
235/45R20 (solid) compared with 235/55R20 (dashed outline), drawn to scale

With 235/55R20 tires, when your speedometer says 60 mph you are really going

63.9 mph

235/55R20 is +1.85 inches (+6.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/45R20235/55R20Difference%
Overall diameter28.33"30.18"+1.85"+6.5%
Section width9.25"9.25"0.00"0.0%
Sidewall height4.16"5.09"+0.93"+22.2%
Circumference88.99"94.80"+5.81"+6.5%
Revolutions per mile712.0668.3−43.7−6.1%

Speedometer difference

If your vehicle came with 235/45R20 and you fit 235/55R20, 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 mph42.6 mph+2.6 mph
50 mph53.3 mph+3.3 mph
60 mph63.9 mph+3.9 mph
65 mph69.2 mph+4.2 mph
70 mph74.6 mph+4.6 mph
75 mph79.9 mph+4.9 mph
80 mph85.2 mph+5.2 mph

Each size on its own

Questions

Which is bigger, 235/45R20 or 235/55R20?
235/55R20 is taller by 1.85 inches (30.18" vs 28.33"). 235/45R20 is the same width.
Can I replace 235/45R20 with 235/55R20?
It is not recommended without checking carefully. The diameter difference is +6.5%, 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 235/55R20 change my speedometer?
When your speedometer reads 60 mph with 235/55R20 fitted (calibrated for 235/45R20), your real speed is 63.9 mph. Your odometer will under-count miles by about 6.5%.
Will 235/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

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