Tire Size Box

255/75R15 vs 255/85R15

255/85R15 compared with 255/75R15

+6.7% diameter, 0.0" width

255/85R15 is 32.07" tall vs 30.06". More than the 3% rule of thumb, so check carefully before switching.

30.1"15" rim7.5"10.0" wide
255/75R15 (solid) compared with 255/85R15 (dashed outline), drawn to scale

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

64.0 mph

255/85R15 is +2.01 inches (+6.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/75R15255/85R15Difference%
Overall diameter30.06"32.07"+2.01"+6.7%
Section width10.04"10.04"0.00"0.0%
Sidewall height7.53"8.53"+1.00"+13.3%
Circumference94.43"100.74"+6.31"+6.7%
Revolutions per mile670.9628.9−42.0−6.3%

Speedometer difference

If your vehicle came with 255/75R15 and you fit 255/85R15, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.7 mph+2.7 mph
50 mph53.3 mph+3.3 mph
60 mph64.0 mph+4.0 mph
65 mph69.3 mph+4.3 mph
70 mph74.7 mph+4.7 mph
75 mph80.0 mph+5.0 mph
80 mph85.3 mph+5.3 mph

Each size on its own

Questions

Which is bigger, 255/75R15 or 255/85R15?
255/85R15 is taller by 2.01 inches (32.07" vs 30.06"). 255/75R15 is the same width.
Can I replace 255/75R15 with 255/85R15?
It is not recommended without checking carefully. The diameter difference is +6.7%, 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 255/75R15), your real speed is 64.0 mph. Your odometer will under-count miles by about 6.7%.
Will 255/85R15 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

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