Tire Size Box

225/45R18 vs 225/55R18

225/55R18 compared with 225/45R18

+6.8% diameter, 0.0" width

225/55R18 is 27.74" tall vs 25.97". More than the 3% rule of thumb, so check carefully before switching.

26.0"18" rim4.0"8.9" wide
225/45R18 (solid) compared with 225/55R18 (dashed outline), drawn to scale

With 225/55R18 tires, when your speedometer says 60 mph you are really going

64.1 mph

225/55R18 is +1.77 inches (+6.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/45R18225/55R18Difference%
Overall diameter25.97"27.74"+1.77"+6.8%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.99"4.87"+0.89"+22.2%
Circumference81.59"87.16"+5.57"+6.8%
Revolutions per mile776.5726.9−49.6−6.4%

Speedometer difference

If your vehicle came with 225/45R18 and you fit 225/55R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.0 mph+2.0 mph
40 mph42.7 mph+2.7 mph
50 mph53.4 mph+3.4 mph
60 mph64.1 mph+4.1 mph
65 mph69.4 mph+4.4 mph
70 mph74.8 mph+4.8 mph
75 mph80.1 mph+5.1 mph
80 mph85.5 mph+5.5 mph

Each size on its own

Questions

Which is bigger, 225/45R18 or 225/55R18?
225/55R18 is taller by 1.77 inches (27.74" vs 25.97"). 225/45R18 is the same width.
Can I replace 225/45R18 with 225/55R18?
It is not recommended without checking carefully. The diameter difference is +6.8%, 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 225/55R18 change my speedometer?
When your speedometer reads 60 mph with 225/55R18 fitted (calibrated for 225/45R18), your real speed is 64.1 mph. Your odometer will under-count miles by about 6.8%.
Will 225/55R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.89 inches of ground clearance at the axle.

Other comparisons