Tire Size Box

275/35R18 vs 275/40R18

275/40R18 compared with 275/35R18

+4.2% diameter, 0.0" width

275/40R18 is 26.66" tall vs 25.58". More than the 3% rule of thumb, so check carefully before switching.

25.6"18" rim3.8"10.8" wide
275/35R18 (solid) compared with 275/40R18 (dashed outline), drawn to scale

With 275/40R18 tires, when your speedometer says 60 mph you are really going

62.5 mph

275/40R18 is +1.08 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

275/35R18275/40R18Difference%
Overall diameter25.58"26.66"+1.08"+4.2%
Section width10.83"10.83"0.00"0.0%
Sidewall height3.79"4.33"+0.54"+14.3%
Circumference80.36"83.76"+3.40"+4.2%
Revolutions per mile788.5756.5−32.0−4.1%

Speedometer difference

If your vehicle came with 275/35R18 and you fit 275/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 275/35R18 or 275/40R18?
275/40R18 is taller by 1.08 inches (26.66" vs 25.58"). 275/35R18 is the same width.
Can I replace 275/35R18 with 275/40R18?
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 275/40R18 change my speedometer?
When your speedometer reads 60 mph with 275/40R18 fitted (calibrated for 275/35R18), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 275/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.54 inches of ground clearance at the axle.

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