Tire Size Box

245/70R18 vs 255/70R18

255/70R18 compared with 245/70R18

+1.7% diameter, +0.4" width

255/70R18 is 32.06" tall vs 31.50". Within the 3% rule, so usually interchangeable.

31.5"18" rim6.8"9.6" wide
245/70R18 (solid) compared with 255/70R18 (dashed outline), drawn to scale

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

61.0 mph

255/70R18 is +0.55 inches (+1.7%) in overall diameter. That is within the 3% most shops and makers allow.

245/70R18255/70R18Difference%
Overall diameter31.50"32.06"+0.55"+1.7%
Section width9.65"10.04"+0.39"+4.1%
Sidewall height6.75"7.03"+0.28"+4.1%
Circumference98.97"100.70"+1.73"+1.7%
Revolutions per mile640.2629.2−11.0−1.7%

Speedometer difference

If your vehicle came with 245/70R18 and you fit 255/70R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.3 mph+0.3 mph
30 mph30.5 mph+0.5 mph
40 mph40.7 mph+0.7 mph
50 mph50.9 mph+0.9 mph
60 mph61.0 mph+1.0 mph
65 mph66.1 mph+1.1 mph
70 mph71.2 mph+1.2 mph
75 mph76.3 mph+1.3 mph
80 mph81.4 mph+1.4 mph

Each size on its own

Questions

Which is bigger, 245/70R18 or 255/70R18?
255/70R18 is taller by 0.55 inches (32.06" vs 31.50"). 255/70R18 is wider by 0.39 inches.
Can I replace 245/70R18 with 255/70R18?
Usually, yes. The diameter difference is +1.7%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 255/70R18 change my speedometer?
When your speedometer reads 60 mph with 255/70R18 fitted (calibrated for 245/70R18), your real speed is 61.0 mph. Your odometer will under-count miles by about 1.7%.
Will 255/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.28 inches of ground clearance at the axle.

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