Tire Size Box

255/35R18 vs 255/40R18

255/40R18 compared with 255/35R18

+4.0% diameter, 0.0" width

255/40R18 is 26.03" tall vs 25.03". More than the 3% rule of thumb, so check carefully before switching.

25.0"18" rim3.5"10.0" wide
255/35R18 (solid) compared with 255/40R18 (dashed outline), drawn to scale

With 255/40R18 tires, when your speedometer says 60 mph you are really going

62.4 mph

255/40R18 is +1.00 inches (+4.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/35R18255/40R18Difference%
Overall diameter25.03"26.03"+1.00"+4.0%
Section width10.04"10.04"0.00"0.0%
Sidewall height3.51"4.02"+0.50"+14.3%
Circumference78.63"81.78"+3.15"+4.0%
Revolutions per mile805.8774.8−31.1−3.9%

Speedometer difference

If your vehicle came with 255/35R18 and you fit 255/40R18, 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, 255/35R18 or 255/40R18?
255/40R18 is taller by 1.00 inches (26.03" vs 25.03"). 255/35R18 is the same width.
Can I replace 255/35R18 with 255/40R18?
It is not recommended without checking carefully. The diameter difference is +4.0%, 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 255/40R18 change my speedometer?
When your speedometer reads 60 mph with 255/40R18 fitted (calibrated for 255/35R18), your real speed is 62.4 mph. Your odometer will under-count miles by about 4.0%.
Will 255/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

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