Tire Size Box

315/65R18 vs 315/70R18

315/70R18 compared with 315/65R18

+3.6% diameter, 0.0" width

315/70R18 is 35.36" tall vs 34.12". More than the 3% rule of thumb, so check carefully before switching.

34.1"18" rim8.1"12.4" wide
315/65R18 (solid) compared with 315/70R18 (dashed outline), drawn to scale

With 315/70R18 tires, when your speedometer says 60 mph you are really going

62.2 mph

315/70R18 is +1.24 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/65R18315/70R18Difference%
Overall diameter34.12"35.36"+1.24"+3.6%
Section width12.40"12.40"0.00"0.0%
Sidewall height8.06"8.68"+0.62"+7.7%
Circumference107.20"111.09"+3.90"+3.6%
Revolutions per mile591.1570.3−20.7−3.5%

Speedometer difference

If your vehicle came with 315/65R18 and you fit 315/70R18, 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 mph41.5 mph+1.5 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.4 mph+2.4 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, 315/65R18 or 315/70R18?
315/70R18 is taller by 1.24 inches (35.36" vs 34.12"). 315/65R18 is the same width.
Can I replace 315/65R18 with 315/70R18?
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 315/70R18 change my speedometer?
When your speedometer reads 60 mph with 315/70R18 fitted (calibrated for 315/65R18), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 315/70R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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