Tire Size Box

305/65R18 vs 305/70R18

305/70R18 compared with 305/65R18

+3.6% diameter, 0.0" width

305/70R18 is 34.81" tall vs 33.61". More than the 3% rule of thumb, so check carefully before switching.

33.6"18" rim7.8"12.0" wide
305/65R18 (solid) compared with 305/70R18 (dashed outline), drawn to scale

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

62.1 mph

305/70R18 is +1.20 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/65R18305/70R18Difference%
Overall diameter33.61"34.81"+1.20"+3.6%
Section width12.01"12.01"0.00"0.0%
Sidewall height7.81"8.41"+0.60"+7.7%
Circumference105.59"109.36"+3.77"+3.6%
Revolutions per mile600.1579.4−20.7−3.4%

Speedometer difference

If your vehicle came with 305/65R18 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 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 305/65R18 or 305/70R18?
305/70R18 is taller by 1.20 inches (34.81" vs 33.61"). 305/65R18 is the same width.
Can I replace 305/65R18 with 305/70R18?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 305/65R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.6%.
Will 305/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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