Tire Size Box

315/35R20 vs 305/40R20

305/40R20 compared with 315/35R20

+3.2% diameter, −0.4" width

305/40R20 is 29.61" 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 305/40R20 (dashed outline), drawn to scale

With 305/40R20 tires, when your speedometer says 60 mph you are really going

61.9 mph

305/40R20 is +0.93 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R20305/40R20Difference%
Overall diameter28.68"29.61"+0.93"+3.2%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.34"4.80"+0.46"+10.7%
Circumference90.10"93.01"+2.91"+3.2%
Revolutions per mile703.2681.2−22.0−3.1%

Speedometer difference

If your vehicle came with 315/35R20 and you fit 305/40R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 315/35R20 or 305/40R20?
305/40R20 is taller by 0.93 inches (29.61" vs 28.68"). 315/35R20 is wider by 0.39 inches.
Can I replace 315/35R20 with 305/40R20?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 305/40R20 change my speedometer?
When your speedometer reads 60 mph with 305/40R20 fitted (calibrated for 315/35R20), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 305/40R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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