Tire Size Box

305/50R18 vs 305/55R18

305/55R18 compared with 305/50R18

+4.0% diameter, 0.0" width

305/55R18 is 31.21" tall vs 30.01". More than the 3% rule of thumb, so check carefully before switching.

30.0"18" rim6.0"12.0" wide
305/50R18 (solid) compared with 305/55R18 (dashed outline), drawn to scale

With 305/55R18 tires, when your speedometer says 60 mph you are really going

62.4 mph

305/55R18 is +1.20 inches (+4.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/50R18305/55R18Difference%
Overall diameter30.01"31.21"+1.20"+4.0%
Section width12.01"12.01"0.00"0.0%
Sidewall height6.00"6.60"+0.60"+10.0%
Circumference94.27"98.04"+3.77"+4.0%
Revolutions per mile672.1646.2−25.9−3.8%

Speedometer difference

If your vehicle came with 305/50R18 and you fit 305/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.2 mph+1.2 mph
40 mph41.6 mph+1.6 mph
50 mph52.0 mph+2.0 mph
60 mph62.4 mph+2.4 mph
65 mph67.6 mph+2.6 mph
70 mph72.8 mph+2.8 mph
75 mph78.0 mph+3.0 mph
80 mph83.2 mph+3.2 mph

Each size on its own

Questions

Which is bigger, 305/50R18 or 305/55R18?
305/55R18 is taller by 1.20 inches (31.21" vs 30.01"). 305/50R18 is the same width.
Can I replace 305/50R18 with 305/55R18?
It is not recommended without checking carefully. The diameter difference is +4.0%, 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 305/55R18 change my speedometer?
When your speedometer reads 60 mph with 305/55R18 fitted (calibrated for 305/50R18), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.0%.
Will 305/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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