Tire Size Box

285/65R18 vs 305/65R18

305/65R18 compared with 285/65R18

+3.1% diameter, +0.8" width

305/65R18 is 33.61" tall vs 32.59". More than the 3% rule of thumb, so check carefully before switching.

32.6"18" rim7.3"11.2" wide
285/65R18 (solid) compared with 305/65R18 (dashed outline), drawn to scale

With 305/65R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

305/65R18 is +1.02 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/65R18305/65R18Difference%
Overall diameter32.59"33.61"+1.02"+3.1%
Section width11.22"12.01"+0.79"+7.0%
Sidewall height7.29"7.81"+0.51"+7.0%
Circumference102.37"105.59"+3.22"+3.1%
Revolutions per mile618.9600.1−18.8−3.0%

Speedometer difference

If your vehicle came with 285/65R18 and you fit 305/65R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 285/65R18 or 305/65R18?
305/65R18 is taller by 1.02 inches (33.61" vs 32.59"). 305/65R18 is wider by 0.79 inches.
Can I replace 285/65R18 with 305/65R18?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 305/65R18 change my speedometer?
When your speedometer reads 60 mph with 305/65R18 fitted (calibrated for 285/65R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.1%.
Will 305/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.51 inches of ground clearance at the axle.

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