Tire Size Box

305/45R18 vs 305/55R18

305/55R18 compared with 305/45R18

+8.3% diameter, 0.0" width

305/55R18 is 31.21" tall vs 28.81". More than the 3% rule of thumb, so check carefully before switching.

28.8"18" rim5.4"12.0" wide
305/45R18 (solid) compared with 305/55R18 (dashed outline), drawn to scale

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

65.0 mph

305/55R18 is +2.40 inches (+8.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R18305/55R18Difference%
Overall diameter28.81"31.21"+2.40"+8.3%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.60"+1.20"+22.2%
Circumference90.50"98.04"+7.54"+8.3%
Revolutions per mile700.1646.2−53.9−7.7%

Speedometer difference

If your vehicle came with 305/45R18 and you fit 305/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.3 mph+6.3 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 305/45R18 or 305/55R18?
305/55R18 is taller by 2.40 inches (31.21" vs 28.81"). 305/45R18 is the same width.
Can I replace 305/45R18 with 305/55R18?
It is not recommended without checking carefully. The diameter difference is +8.3%, 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 305/45R18), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.3%.
Will 305/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

Other comparisons