Tire Size Box

285/60R18 vs 305/60R18

305/60R18 compared with 285/60R18

+3.0% diameter, +0.8" width

305/60R18 is 32.41" tall vs 31.46". More than the 3% rule of thumb, so check carefully before switching.

31.5"18" rim6.7"11.2" wide
285/60R18 (solid) compared with 305/60R18 (dashed outline), drawn to scale

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

61.8 mph

305/60R18 is +0.94 inches (+3.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/60R18305/60R18Difference%
Overall diameter31.46"32.41"+0.94"+3.0%
Section width11.22"12.01"+0.79"+7.0%
Sidewall height6.73"7.20"+0.47"+7.0%
Circumference98.85"101.82"+2.97"+3.0%
Revolutions per mile641.0622.3−18.7−2.9%

Speedometer difference

If your vehicle came with 285/60R18 and you fit 305/60R18, 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 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.4 mph+2.4 mph

Each size on its own

Questions

Which is bigger, 285/60R18 or 305/60R18?
305/60R18 is taller by 0.94 inches (32.41" vs 31.46"). 305/60R18 is wider by 0.79 inches.
Can I replace 285/60R18 with 305/60R18?
It is not recommended without checking carefully. The diameter difference is +3.0%, 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 305/60R18 change my speedometer?
When your speedometer reads 60 mph with 305/60R18 fitted (calibrated for 285/60R18), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.0%.
Will 305/60R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.47 inches of ground clearance at the axle.

Other comparisons