Tire Size Box

315/55R18 vs 315/65R18

315/65R18 compared with 315/55R18

+7.8% diameter, 0.0" width

315/65R18 is 34.12" tall vs 31.64". More than the 3% rule of thumb, so check carefully before switching.

31.6"18" rim6.8"12.4" wide
315/55R18 (solid) compared with 315/65R18 (dashed outline), drawn to scale

With 315/65R18 tires, when your speedometer says 60 mph you are really going

64.7 mph

315/65R18 is +2.48 inches (+7.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/55R18315/65R18Difference%
Overall diameter31.64"34.12"+2.48"+7.8%
Section width12.40"12.40"0.00"0.0%
Sidewall height6.82"8.06"+1.24"+18.2%
Circumference99.41"107.20"+7.79"+7.8%
Revolutions per mile637.4591.1−46.3−7.3%

Speedometer difference

If your vehicle came with 315/55R18 and you fit 315/65R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.4 mph+2.4 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.9 mph+5.9 mph
80 mph86.3 mph+6.3 mph

Each size on its own

Questions

Which is bigger, 315/55R18 or 315/65R18?
315/65R18 is taller by 2.48 inches (34.12" vs 31.64"). 315/55R18 is the same width.
Can I replace 315/55R18 with 315/65R18?
It is not recommended without checking carefully. The diameter difference is +7.8%, 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 315/65R18 change my speedometer?
When your speedometer reads 60 mph with 315/65R18 fitted (calibrated for 315/55R18), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.8%.
Will 315/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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