Tire Size Box

315/25R20 vs 315/30R20

315/30R20 compared with 315/25R20

+4.7% diameter, 0.0" width

315/30R20 is 27.44" tall vs 26.20". More than the 3% rule of thumb, so check carefully before switching.

26.2"20" rim3.1"12.4" wide
315/25R20 (solid) compared with 315/30R20 (dashed outline), drawn to scale

With 315/30R20 tires, when your speedometer says 60 mph you are really going

62.8 mph

315/30R20 is +1.24 inches (+4.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R20315/30R20Difference%
Overall diameter26.20"27.44"+1.24"+4.7%
Section width12.40"12.40"0.00"0.0%
Sidewall height3.10"3.72"+0.62"+20.0%
Circumference82.31"86.21"+3.90"+4.7%
Revolutions per mile769.8735.0−34.8−4.5%

Speedometer difference

If your vehicle came with 315/25R20 and you fit 315/30R20, 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 mph52.4 mph+2.4 mph
60 mph62.8 mph+2.8 mph
65 mph68.1 mph+3.1 mph
70 mph73.3 mph+3.3 mph
75 mph78.5 mph+3.5 mph
80 mph83.8 mph+3.8 mph

Each size on its own

Questions

Which is bigger, 315/25R20 or 315/30R20?
315/30R20 is taller by 1.24 inches (27.44" vs 26.20"). 315/25R20 is the same width.
Can I replace 315/25R20 with 315/30R20?
It is not recommended without checking carefully. The diameter difference is +4.7%, 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 315/30R20 change my speedometer?
When your speedometer reads 60 mph with 315/30R20 fitted (calibrated for 315/25R20), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.7%.
Will 315/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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