Tire Size Box

245/45R20 vs 245/55R20

245/55R20 compared with 245/45R20

+6.7% diameter, 0.0" width

245/55R20 is 30.61" tall vs 28.68". More than the 3% rule of thumb, so check carefully before switching.

28.7"20" rim4.3"9.6" wide
245/45R20 (solid) compared with 245/55R20 (dashed outline), drawn to scale

With 245/55R20 tires, when your speedometer says 60 mph you are really going

64.0 mph

245/55R20 is +1.93 inches (+6.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

245/45R20245/55R20Difference%
Overall diameter28.68"30.61"+1.93"+6.7%
Section width9.65"9.65"0.00"0.0%
Sidewall height4.34"5.31"+0.96"+22.2%
Circumference90.10"96.16"+6.06"+6.7%
Revolutions per mile703.2658.9−44.3−6.3%

Speedometer difference

If your vehicle came with 245/45R20 and you fit 245/55R20, 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 mph42.7 mph+2.7 mph
50 mph53.4 mph+3.4 mph
60 mph64.0 mph+4.0 mph
65 mph69.4 mph+4.4 mph
70 mph74.7 mph+4.7 mph
75 mph80.0 mph+5.0 mph
80 mph85.4 mph+5.4 mph

Each size on its own

Questions

Which is bigger, 245/45R20 or 245/55R20?
245/55R20 is taller by 1.93 inches (30.61" vs 28.68"). 245/45R20 is the same width.
Can I replace 245/45R20 with 245/55R20?
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 245/55R20 change my speedometer?
When your speedometer reads 60 mph with 245/55R20 fitted (calibrated for 245/45R20), your real speed is 64.0 mph. Your odometer will under-count miles by about 6.7%.
Will 245/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.96 inches of ground clearance at the axle.

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