Tire Size Box

315/30R20 vs 315/40R20

315/40R20 compared with 315/30R20

+9.0% diameter, 0.0" width

315/40R20 is 29.92" tall vs 27.44". More than the 3% rule of thumb, so check carefully before switching.

27.4"20" rim3.7"12.4" wide
315/30R20 (solid) compared with 315/40R20 (dashed outline), drawn to scale

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

65.4 mph

315/40R20 is +2.48 inches (+9.0%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/30R20315/40R20Difference%
Overall diameter27.44"29.92"+2.48"+9.0%
Section width12.40"12.40"0.00"0.0%
Sidewall height3.72"4.96"+1.24"+33.3%
Circumference86.21"94.00"+7.79"+9.0%
Revolutions per mile735.0674.0−60.9−8.3%

Speedometer difference

If your vehicle came with 315/30R20 and you fit 315/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.5 mph+4.5 mph
60 mph65.4 mph+5.4 mph
65 mph70.9 mph+5.9 mph
70 mph76.3 mph+6.3 mph
75 mph81.8 mph+6.8 mph
80 mph87.2 mph+7.2 mph

Each size on its own

Questions

Which is bigger, 315/30R20 or 315/40R20?
315/40R20 is taller by 2.48 inches (29.92" vs 27.44"). 315/30R20 is the same width.
Can I replace 315/30R20 with 315/40R20?
It is not recommended without checking carefully. The diameter difference is +9.0%, 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 315/30R20), your real speed is 65.4 mph. Your odometer will under-count miles by about 9.0%.
Will 315/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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