Tire Size Box

315/35R20 vs 315/40R20

315/40R20 compared with 315/35R20

+4.3% diameter, 0.0" width

315/40R20 is 29.92" tall vs 28.68". More than the 3% rule of thumb, so check carefully before switching.

28.7"20" rim4.3"12.4" wide
315/35R20 (solid) compared with 315/40R20 (dashed outline), drawn to scale

With 315/40R20 tires, when your speedometer says 60 mph you are really going

62.6 mph

315/40R20 is +1.24 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R20315/40R20Difference%
Overall diameter28.68"29.92"+1.24"+4.3%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.34"4.96"+0.62"+14.3%
Circumference90.10"94.00"+3.90"+4.3%
Revolutions per mile703.2674.0−29.1−4.1%

Speedometer difference

If your vehicle came with 315/35R20 and you fit 315/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 315/35R20 or 315/40R20?
315/40R20 is taller by 1.24 inches (29.92" vs 28.68"). 315/35R20 is the same width.
Can I replace 315/35R20 with 315/40R20?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 315/40R20 change my speedometer?
When your speedometer reads 60 mph with 315/40R20 fitted (calibrated for 315/35R20), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 315/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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