Tire Size Box

275/40R18 vs 275/45R18

275/45R18 compared with 275/40R18

+4.1% diameter, 0.0" width

275/45R18 is 27.74" tall vs 26.66". More than the 3% rule of thumb, so check carefully before switching.

26.7"18" rim4.3"10.8" wide
275/40R18 (solid) compared with 275/45R18 (dashed outline), drawn to scale

With 275/45R18 tires, when your speedometer says 60 mph you are really going

62.4 mph

275/45R18 is +1.08 inches (+4.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/40R18275/45R18Difference%
Overall diameter26.66"27.74"+1.08"+4.1%
Section width10.83"10.83"0.00"0.0%
Sidewall height4.33"4.87"+0.54"+12.5%
Circumference83.76"87.16"+3.40"+4.1%
Revolutions per mile756.5726.9−29.5−3.9%

Speedometer difference

If your vehicle came with 275/40R18 and you fit 275/45R18, 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 mph41.6 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, 275/40R18 or 275/45R18?
275/45R18 is taller by 1.08 inches (27.74" vs 26.66"). 275/40R18 is the same width.
Can I replace 275/40R18 with 275/45R18?
It is not recommended without checking carefully. The diameter difference is +4.1%, 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 275/45R18 change my speedometer?
When your speedometer reads 60 mph with 275/45R18 fitted (calibrated for 275/40R18), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.1%.
Will 275/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

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