Tire Size Box

305/65R20 vs 335/55R21

335/55R21 compared with 305/65R20

−0.3% diameter, +1.2" width

335/55R21 is 35.51" tall vs 35.61". Within the 3% rule, so usually interchangeable.

35.6"20" rim7.8"12.0" wide
305/65R20 (solid) compared with 335/55R21 (dashed outline), drawn to scale

With 335/55R21 tires, when your speedometer says 60 mph you are really going

59.8 mph

335/55R21 is −0.10 inches (−0.3%) in overall diameter. That is within the 3% most shops and makers allow.

305/65R20335/55R21Difference%
Overall diameter35.61"35.51"−0.10"−0.3%
Section width12.01"13.19"+1.18"+9.8%
Sidewall height7.81"7.25"−0.55"−7.1%
Circumference111.87"111.55"−0.32"−0.3%
Revolutions per mile566.4568.0+1.6+0.3%

Speedometer difference

If your vehicle came with 305/65R20 and you fit 335/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph19.9 mph−0.1 mph
30 mph29.9 mph−0.1 mph
40 mph39.9 mph−0.1 mph
50 mph49.9 mph−0.1 mph
60 mph59.8 mph−0.2 mph
65 mph64.8 mph−0.2 mph
70 mph69.8 mph−0.2 mph
75 mph74.8 mph−0.2 mph
80 mph79.8 mph−0.2 mph

Each size on its own

Questions

Which is bigger, 305/65R20 or 335/55R21?
305/65R20 is taller by 0.10 inches (35.61" vs 35.51"). 335/55R21 is wider by 1.18 inches.
Can I replace 305/65R20 with 335/55R21?
Usually, yes. The diameter difference is −0.3%, 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 335/55R21 change my speedometer?
When your speedometer reads 60 mph with 335/55R21 fitted (calibrated for 305/65R20), your real speed is 59.8 mph. Your odometer will over-count miles by about 0.3%.
Will 335/55R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.05 inches of ground clearance at the axle.

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