Tire Size Box

235/70R18 vs 255/70R18

255/70R18 compared with 235/70R18

+3.6% diameter, +0.8" width

255/70R18 is 32.06" tall vs 30.95". More than the 3% rule of thumb, so check carefully before switching.

31.0"18" rim6.5"9.3" wide
235/70R18 (solid) compared with 255/70R18 (dashed outline), drawn to scale

With 255/70R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

255/70R18 is +1.10 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/70R18255/70R18Difference%
Overall diameter30.95"32.06"+1.10"+3.6%
Section width9.25"10.04"+0.79"+8.5%
Sidewall height6.48"7.03"+0.55"+8.5%
Circumference97.24"100.70"+3.46"+3.6%
Revolutions per mile651.6629.2−22.4−3.4%

Speedometer difference

If your vehicle came with 235/70R18 and you fit 255/70R18, 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 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 235/70R18 or 255/70R18?
255/70R18 is taller by 1.10 inches (32.06" vs 30.95"). 255/70R18 is wider by 0.79 inches.
Can I replace 235/70R18 with 255/70R18?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 255/70R18 change my speedometer?
When your speedometer reads 60 mph with 255/70R18 fitted (calibrated for 235/70R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 255/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.55 inches of ground clearance at the axle.

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