Tire Size Box

285/70R18 vs 305/70R18

305/70R18 compared with 285/70R18

+3.3% diameter, +0.8" width

305/70R18 is 34.81" tall vs 33.71". More than the 3% rule of thumb, so check carefully before switching.

33.7"18" rim7.9"11.2" wide
285/70R18 (solid) compared with 305/70R18 (dashed outline), drawn to scale

With 305/70R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

305/70R18 is +1.10 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

285/70R18305/70R18Difference%
Overall diameter33.71"34.81"+1.10"+3.3%
Section width11.22"12.01"+0.79"+7.0%
Sidewall height7.85"8.41"+0.55"+7.0%
Circumference105.90"109.36"+3.46"+3.3%
Revolutions per mile598.3579.4−18.9−3.2%

Speedometer difference

If your vehicle came with 285/70R18 and you fit 305/70R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 285/70R18 or 305/70R18?
305/70R18 is taller by 1.10 inches (34.81" vs 33.71"). 305/70R18 is wider by 0.79 inches.
Can I replace 285/70R18 with 305/70R18?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 305/70R18 change my speedometer?
When your speedometer reads 60 mph with 305/70R18 fitted (calibrated for 285/70R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 305/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.55 inches of ground clearance at the axle.

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