Tire Size Box

315/25R20 vs 305/30R20

305/30R20 compared with 315/25R20

+3.8% diameter, −0.4" width

305/30R20 is 27.20" 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 305/30R20 (dashed outline), drawn to scale

With 305/30R20 tires, when your speedometer says 60 mph you are really going

62.3 mph

305/30R20 is +1.00 inches (+3.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R20305/30R20Difference%
Overall diameter26.20"27.20"+1.00"+3.8%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height3.10"3.60"+0.50"+16.2%
Circumference82.31"85.47"+3.15"+3.8%
Revolutions per mile769.8741.3−28.4−3.7%

Speedometer difference

If your vehicle came with 315/25R20 and you fit 305/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.1 mph+1.1 mph
40 mph41.5 mph+1.5 mph
50 mph51.9 mph+1.9 mph
60 mph62.3 mph+2.3 mph
65 mph67.5 mph+2.5 mph
70 mph72.7 mph+2.7 mph
75 mph77.9 mph+2.9 mph
80 mph83.1 mph+3.1 mph

Each size on its own

Questions

Which is bigger, 315/25R20 or 305/30R20?
305/30R20 is taller by 1.00 inches (27.20" vs 26.20"). 315/25R20 is wider by 0.39 inches.
Can I replace 315/25R20 with 305/30R20?
It is not recommended without checking carefully. The diameter difference is +3.8%, 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 305/30R20 change my speedometer?
When your speedometer reads 60 mph with 305/30R20 fitted (calibrated for 315/25R20), your real speed is 62.3 mph. Your odometer will under-count miles by about 3.8%.
Will 305/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

Other comparisons