Tire Size Box

235/60R18 vs 255/60R18

255/60R18 compared with 235/60R18

+3.2% diameter, +0.8" width

255/60R18 is 30.05" tall vs 29.10". More than the 3% rule of thumb, so check carefully before switching.

29.1"18" rim5.6"9.3" wide
235/60R18 (solid) compared with 255/60R18 (dashed outline), drawn to scale

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

61.9 mph

255/60R18 is +0.94 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/60R18255/60R18Difference%
Overall diameter29.10"30.05"+0.94"+3.2%
Section width9.25"10.04"+0.79"+8.5%
Sidewall height5.55"6.02"+0.47"+8.5%
Circumference91.43"94.40"+2.97"+3.2%
Revolutions per mile693.0671.2−21.8−3.1%

Speedometer difference

If your vehicle came with 235/60R18 and you fit 255/60R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 235/60R18 or 255/60R18?
255/60R18 is taller by 0.94 inches (30.05" vs 29.10"). 255/60R18 is wider by 0.79 inches.
Can I replace 235/60R18 with 255/60R18?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 255/60R18 change my speedometer?
When your speedometer reads 60 mph with 255/60R18 fitted (calibrated for 235/60R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 255/60R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.47 inches of ground clearance at the axle.

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