Tire Size Box

255/40R18 vs 255/45R18

255/45R18 compared with 255/40R18

+3.9% diameter, 0.0" width

255/45R18 is 27.04" tall vs 26.03". More than the 3% rule of thumb, so check carefully before switching.

26.0"18" rim4.0"10.0" wide
255/40R18 (solid) compared with 255/45R18 (dashed outline), drawn to scale

With 255/45R18 tires, when your speedometer says 60 mph you are really going

62.3 mph

255/45R18 is +1.00 inches (+3.9%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/40R18255/45R18Difference%
Overall diameter26.03"27.04"+1.00"+3.9%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.02"4.52"+0.50"+12.5%
Circumference81.78"84.93"+3.15"+3.9%
Revolutions per mile774.8746.0−28.8−3.7%

Speedometer difference

If your vehicle came with 255/40R18 and you fit 255/45R18, 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 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 255/40R18 or 255/45R18?
255/45R18 is taller by 1.00 inches (27.04" vs 26.03"). 255/40R18 is the same width.
Can I replace 255/40R18 with 255/45R18?
It is not recommended without checking carefully. The diameter difference is +3.9%, 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 255/45R18 change my speedometer?
When your speedometer reads 60 mph with 255/45R18 fitted (calibrated for 255/40R18), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.9%.
Will 255/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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