Tire Size Box

235/75R18 vs 255/75R18

255/75R18 compared with 235/75R18

+3.7% diameter, +0.8" width

255/75R18 is 33.06" tall vs 31.88". More than the 3% rule of thumb, so check carefully before switching.

31.9"18" rim6.9"9.3" wide
235/75R18 (solid) compared with 255/75R18 (dashed outline), drawn to scale

With 255/75R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

255/75R18 is +1.18 inches (+3.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/75R18255/75R18Difference%
Overall diameter31.88"33.06"+1.18"+3.7%
Section width9.25"10.04"+0.79"+8.5%
Sidewall height6.94"7.53"+0.59"+8.5%
Circumference100.15"103.86"+3.71"+3.7%
Revolutions per mile632.7610.1−22.6−3.6%

Speedometer difference

If your vehicle came with 235/75R18 and you fit 255/75R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 mph
70 mph72.6 mph+2.6 mph
75 mph77.8 mph+2.8 mph
80 mph83.0 mph+3.0 mph

Each size on its own

Questions

Which is bigger, 235/75R18 or 255/75R18?
255/75R18 is taller by 1.18 inches (33.06" vs 31.88"). 255/75R18 is wider by 0.79 inches.
Can I replace 235/75R18 with 255/75R18?
It is not recommended without checking carefully. The diameter difference is +3.7%, 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 255/75R18 change my speedometer?
When your speedometer reads 60 mph with 255/75R18 fitted (calibrated for 235/75R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.7%.
Will 255/75R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.59 inches of ground clearance at the axle.

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