Tire Size Box

315/30R20 vs 305/35R20

305/35R20 compared with 315/30R20

+3.5% diameter, −0.4" width

305/35R20 is 28.41" 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 305/35R20 (dashed outline), drawn to scale

With 305/35R20 tires, when your speedometer says 60 mph you are really going

62.1 mph

305/35R20 is +0.96 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/30R20305/35R20Difference%
Overall diameter27.44"28.41"+0.96"+3.5%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height3.72"4.20"+0.48"+13.0%
Circumference86.21"89.24"+3.03"+3.5%
Revolutions per mile735.0710.0−25.0−3.4%

Speedometer difference

If your vehicle came with 315/30R20 and you fit 305/35R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 315/30R20 or 305/35R20?
305/35R20 is taller by 0.96 inches (28.41" vs 27.44"). 315/30R20 is wider by 0.39 inches.
Can I replace 315/30R20 with 305/35R20?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 305/35R20 change my speedometer?
When your speedometer reads 60 mph with 305/35R20 fitted (calibrated for 315/30R20), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 305/35R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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