Tire Size Box

285/40R18 vs 285/45R18

285/45R18 compared with 285/40R18

+4.2% diameter, 0.0" width

285/45R18 is 28.10" tall vs 26.98". More than the 3% rule of thumb, so check carefully before switching.

27.0"18" rim4.5"11.2" wide
285/40R18 (solid) compared with 285/45R18 (dashed outline), drawn to scale

With 285/45R18 tires, when your speedometer says 60 mph you are really going

62.5 mph

285/45R18 is +1.12 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/40R18285/45R18Difference%
Overall diameter26.98"28.10"+1.12"+4.2%
Section width11.22"11.22"0.00"0.0%
Sidewall height4.49"5.05"+0.56"+12.5%
Circumference84.75"88.27"+3.53"+4.2%
Revolutions per mile747.6717.8−29.9−4.0%

Speedometer difference

If your vehicle came with 285/40R18 and you fit 285/45R18, 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 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 285/40R18 or 285/45R18?
285/45R18 is taller by 1.12 inches (28.10" vs 26.98"). 285/40R18 is the same width.
Can I replace 285/40R18 with 285/45R18?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 285/45R18 change my speedometer?
When your speedometer reads 60 mph with 285/45R18 fitted (calibrated for 285/40R18), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 285/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.56 inches of ground clearance at the axle.

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