Tire Size Box

315/35R18 vs 315/40R18

315/40R18 compared with 315/35R18

+4.6% diameter, 0.0" width

315/40R18 is 27.92" tall vs 26.68". More than the 3% rule of thumb, so check carefully before switching.

26.7"18" rim4.3"12.4" wide
315/35R18 (solid) compared with 315/40R18 (dashed outline), drawn to scale

With 315/40R18 tires, when your speedometer says 60 mph you are really going

62.8 mph

315/40R18 is +1.24 inches (+4.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R18315/40R18Difference%
Overall diameter26.68"27.92"+1.24"+4.6%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.34"4.96"+0.62"+14.3%
Circumference83.82"87.72"+3.90"+4.6%
Revolutions per mile755.9722.3−33.6−4.4%

Speedometer difference

If your vehicle came with 315/35R18 and you fit 315/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.9 mph+1.9 mph
50 mph52.3 mph+2.3 mph
60 mph62.8 mph+2.8 mph
65 mph68.0 mph+3.0 mph
70 mph73.3 mph+3.3 mph
75 mph78.5 mph+3.5 mph
80 mph83.7 mph+3.7 mph

Each size on its own

Questions

Which is bigger, 315/35R18 or 315/40R18?
315/40R18 is taller by 1.24 inches (27.92" vs 26.68"). 315/35R18 is the same width.
Can I replace 315/35R18 with 315/40R18?
It is not recommended without checking carefully. The diameter difference is +4.6%, 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 315/40R18 change my speedometer?
When your speedometer reads 60 mph with 315/40R18 fitted (calibrated for 315/35R18), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.6%.
Will 315/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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