Tire Size Box

255/65R18 vs 255/70R18

255/70R18 compared with 255/65R18

+3.2% diameter, 0.0" width

255/70R18 is 32.06" tall vs 31.05". More than the 3% rule of thumb, so check carefully before switching.

31.1"18" rim6.5"10.0" wide
255/65R18 (solid) compared with 255/70R18 (dashed outline), drawn to scale

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

61.9 mph

255/70R18 is +1.00 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/65R18255/70R18Difference%
Overall diameter31.05"32.06"+1.00"+3.2%
Section width10.04"10.04"0.00"0.0%
Sidewall height6.53"7.03"+0.50"+7.7%
Circumference97.55"100.70"+3.15"+3.2%
Revolutions per mile649.5629.2−20.3−3.1%

Speedometer difference

If your vehicle came with 255/65R18 and you fit 255/70R18, 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, 255/65R18 or 255/70R18?
255/70R18 is taller by 1.00 inches (32.06" vs 31.05"). 255/65R18 is the same width.
Can I replace 255/65R18 with 255/70R18?
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 255/70R18 change my speedometer?
When your speedometer reads 60 mph with 255/70R18 fitted (calibrated for 255/65R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 255/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

Other comparisons