Tire Size Box

255/25R20 vs 245/30R20

245/30R20 compared with 255/25R20

+3.1% diameter, −0.4" width

245/30R20 is 25.79" tall vs 25.02". More than the 3% rule of thumb, so check carefully before switching.

25.0"20" rim2.5"10.0" wide
255/25R20 (solid) compared with 245/30R20 (dashed outline), drawn to scale

With 245/30R20 tires, when your speedometer says 60 mph you are really going

61.8 mph

245/30R20 is +0.77 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/25R20245/30R20Difference%
Overall diameter25.02"25.79"+0.77"+3.1%
Section width10.04"9.65"−0.39"−3.9%
Sidewall height2.51"2.89"+0.38"+15.3%
Circumference78.60"81.01"+2.41"+3.1%
Revolutions per mile806.1782.1−24.0−3.0%

Speedometer difference

If your vehicle came with 255/25R20 and you fit 245/30R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 255/25R20 or 245/30R20?
245/30R20 is taller by 0.77 inches (25.79" vs 25.02"). 255/25R20 is wider by 0.39 inches.
Can I replace 255/25R20 with 245/30R20?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 245/30R20 change my speedometer?
When your speedometer reads 60 mph with 245/30R20 fitted (calibrated for 255/25R20), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 245/30R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.38 inches of ground clearance at the axle.

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