Tire Size Box

235/85R15 vs 255/85R15

255/85R15 compared with 235/85R15

+4.4% diameter, +0.8" width

255/85R15 is 32.07" tall vs 30.73". More than the 3% rule of thumb, so check carefully before switching.

30.7"15" rim7.9"9.3" wide
235/85R15 (solid) compared with 255/85R15 (dashed outline), drawn to scale

With 255/85R15 tires, when your speedometer says 60 mph you are really going

62.6 mph

255/85R15 is +1.34 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/85R15255/85R15Difference%
Overall diameter30.73"32.07"+1.34"+4.4%
Section width9.25"10.04"+0.79"+8.5%
Sidewall height7.86"8.53"+0.67"+8.5%
Circumference96.54"100.74"+4.21"+4.4%
Revolutions per mile656.3628.9−27.4−4.2%

Speedometer difference

If your vehicle came with 235/85R15 and you fit 255/85R15, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 235/85R15 or 255/85R15?
255/85R15 is taller by 1.34 inches (32.07" vs 30.73"). 255/85R15 is wider by 0.79 inches.
Can I replace 235/85R15 with 255/85R15?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 255/85R15 change my speedometer?
When your speedometer reads 60 mph with 255/85R15 fitted (calibrated for 235/85R15), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.4%.
Will 255/85R15 lift my vehicle?
Ride height changes by half the diameter difference: +0.67 inches of ground clearance at the axle.

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