Tire Size Box

275/45R18 vs 275/55R18

275/55R18 compared with 275/45R18

+7.8% diameter, 0.0" width

275/55R18 is 29.91" tall vs 27.74". More than the 3% rule of thumb, so check carefully before switching.

27.7"18" rim4.9"10.8" wide
275/45R18 (solid) compared with 275/55R18 (dashed outline), drawn to scale

With 275/55R18 tires, when your speedometer says 60 mph you are really going

64.7 mph

275/55R18 is +2.17 inches (+7.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/45R18275/55R18Difference%
Overall diameter27.74"29.91"+2.17"+7.8%
Section width10.83"10.83"0.00"0.0%
Sidewall height4.87"5.95"+1.08"+22.2%
Circumference87.16"93.96"+6.80"+7.8%
Revolutions per mile726.9674.3−52.6−7.2%

Speedometer difference

If your vehicle came with 275/45R18 and you fit 275/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 275/45R18 or 275/55R18?
275/55R18 is taller by 2.17 inches (29.91" vs 27.74"). 275/45R18 is the same width.
Can I replace 275/45R18 with 275/55R18?
It is not recommended without checking carefully. The diameter difference is +7.8%, 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 275/55R18 change my speedometer?
When your speedometer reads 60 mph with 275/55R18 fitted (calibrated for 275/45R18), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.8%.
Will 275/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.08 inches of ground clearance at the axle.

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