Tire Size Box

305/30R20 vs 305/35R20

305/35R20 compared with 305/30R20

+4.4% diameter, 0.0" width

305/35R20 is 28.41" tall vs 27.20". More than the 3% rule of thumb, so check carefully before switching.

27.2"20" rim3.6"12.0" wide
305/30R20 (solid) compared with 305/35R20 (dashed outline), drawn to scale

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

62.6 mph

305/35R20 is +1.20 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R20305/35R20Difference%
Overall diameter27.20"28.41"+1.20"+4.4%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.20"+0.60"+16.7%
Circumference85.47"89.24"+3.77"+4.4%
Revolutions per mile741.3710.0−31.3−4.2%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.6 mph+2.6 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.5 mph+3.5 mph

Each size on its own

Questions

Which is bigger, 305/30R20 or 305/35R20?
305/35R20 is taller by 1.20 inches (28.41" vs 27.20"). 305/30R20 is the same width.
Can I replace 305/30R20 with 305/35R20?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 305/30R20), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.4%.
Will 305/35R20 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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