Tire Size Box

205/45R18 vs 205/55R18

205/55R18 compared with 205/45R18

+6.4% diameter, 0.0" width

205/55R18 is 26.88" tall vs 25.26". More than the 3% rule of thumb, so check carefully before switching.

25.3"18" rim3.6"8.1" wide
205/45R18 (solid) compared with 205/55R18 (dashed outline), drawn to scale

With 205/55R18 tires, when your speedometer says 60 mph you are really going

63.8 mph

205/55R18 is +1.61 inches (+6.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

205/45R18205/55R18Difference%
Overall diameter25.26"26.88"+1.61"+6.4%
Section width8.07"8.07"0.00"0.0%
Sidewall height3.63"4.44"+0.81"+22.2%
Circumference79.37"84.44"+5.07"+6.4%
Revolutions per mile798.3750.4−47.9−6.0%

Speedometer difference

If your vehicle came with 205/45R18 and you fit 205/55R18, 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 mph42.6 mph+2.6 mph
50 mph53.2 mph+3.2 mph
60 mph63.8 mph+3.8 mph
65 mph69.2 mph+4.2 mph
70 mph74.5 mph+4.5 mph
75 mph79.8 mph+4.8 mph
80 mph85.1 mph+5.1 mph

Each size on its own

Questions

Which is bigger, 205/45R18 or 205/55R18?
205/55R18 is taller by 1.61 inches (26.88" vs 25.26"). 205/45R18 is the same width.
Can I replace 205/45R18 with 205/55R18?
It is not recommended without checking carefully. The diameter difference is +6.4%, 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 205/55R18 change my speedometer?
When your speedometer reads 60 mph with 205/55R18 fitted (calibrated for 205/45R18), your real speed is 63.8 mph. Your odometer will under-count miles by about 6.4%.
Will 205/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.81 inches of ground clearance at the axle.

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