Tire Size Box

255/30R20 vs 245/35R20

245/35R20 compared with 255/30R20

+2.8% diameter, −0.4" width

245/35R20 is 26.75" tall vs 26.02". Within the 3% rule, so usually interchangeable.

26.0"20" rim3.0"10.0" wide
255/30R20 (solid) compared with 245/35R20 (dashed outline), drawn to scale

With 245/35R20 tires, when your speedometer says 60 mph you are really going

61.7 mph

245/35R20 is +0.73 inches (+2.8%) in overall diameter. That is within the 3% most shops and makers allow.

255/30R20245/35R20Difference%
Overall diameter26.02"26.75"+0.73"+2.8%
Section width10.04"9.65"−0.39"−3.9%
Sidewall height3.01"3.38"+0.36"+12.1%
Circumference81.76"84.04"+2.29"+2.8%
Revolutions per mile775.0753.9−21.1−2.7%

Speedometer difference

If your vehicle came with 255/30R20 and you fit 245/35R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.8 mph+0.8 mph
40 mph41.1 mph+1.1 mph
50 mph51.4 mph+1.4 mph
60 mph61.7 mph+1.7 mph
65 mph66.8 mph+1.8 mph
70 mph72.0 mph+2.0 mph
75 mph77.1 mph+2.1 mph
80 mph82.2 mph+2.2 mph

Each size on its own

Questions

Which is bigger, 255/30R20 or 245/35R20?
245/35R20 is taller by 0.73 inches (26.75" vs 26.02"). 255/30R20 is wider by 0.39 inches.
Can I replace 255/30R20 with 245/35R20?
Usually, yes. The diameter difference is +2.8%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 245/35R20 change my speedometer?
When your speedometer reads 60 mph with 245/35R20 fitted (calibrated for 255/30R20), your real speed is 61.7 mph. Your odometer will under-count miles by about 2.8%.
Will 245/35R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.36 inches of ground clearance at the axle.

Other comparisons