Tire Size Box

305/30R21 vs 305/35R21

305/35R21 compared with 305/30R21

+4.3% diameter, 0.0" width

305/35R21 is 29.41" tall vs 28.20". More than the 3% rule of thumb, so check carefully before switching.

28.2"21" rim3.6"12.0" wide
305/30R21 (solid) compared with 305/35R21 (dashed outline), drawn to scale

With 305/35R21 tires, when your speedometer says 60 mph you are really going

62.6 mph

305/35R21 is +1.20 inches (+4.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/30R21305/35R21Difference%
Overall diameter28.20"29.41"+1.20"+4.3%
Section width12.01"12.01"0.00"0.0%
Sidewall height3.60"4.20"+0.60"+16.7%
Circumference88.61"92.38"+3.77"+4.3%
Revolutions per mile715.1685.9−29.2−4.1%

Speedometer difference

If your vehicle came with 305/30R21 and you fit 305/35R21, 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 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.6 mph+2.6 mph
65 mph67.8 mph+2.8 mph
70 mph73.0 mph+3.0 mph
75 mph78.2 mph+3.2 mph
80 mph83.4 mph+3.4 mph

Each size on its own

Questions

Which is bigger, 305/30R21 or 305/35R21?
305/35R21 is taller by 1.20 inches (29.41" vs 28.20"). 305/30R21 is the same width.
Can I replace 305/30R21 with 305/35R21?
It is not recommended without checking carefully. The diameter difference is +4.3%, 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 305/35R21 change my speedometer?
When your speedometer reads 60 mph with 305/35R21 fitted (calibrated for 305/30R21), your real speed is 62.6 mph. Your odometer will under-count miles by about 4.3%.
Will 305/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.60 inches of ground clearance at the axle.

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