Tire Size Box

315/55R20 vs 355/45R21

355/45R21 compared with 315/55R20

−0.2% diameter, +1.6" width

355/45R21 is 33.58" tall vs 33.64". Within the 3% rule, so usually interchangeable.

33.6"20" rim6.8"12.4" wide
315/55R20 (solid) compared with 355/45R21 (dashed outline), drawn to scale

With 355/45R21 tires, when your speedometer says 60 mph you are really going

59.9 mph

355/45R21 is −0.06 inches (−0.2%) in overall diameter. That is within the 3% most shops and makers allow.

315/55R20355/45R21Difference%
Overall diameter33.64"33.58"−0.06"−0.2%
Section width12.40"13.98"+1.57"+12.7%
Sidewall height6.82"6.29"−0.53"−7.8%
Circumference105.69"105.49"−0.20"−0.2%
Revolutions per mile599.5600.6+1.1+0.2%

Speedometer difference

If your vehicle came with 315/55R20 and you fit 355/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph29.9 mph−0.1 mph
40 mph39.9 mph−0.1 mph
50 mph49.9 mph−0.1 mph
60 mph59.9 mph−0.1 mph
65 mph64.9 mph−0.1 mph
70 mph69.9 mph−0.1 mph
75 mph74.9 mph−0.1 mph
80 mph79.9 mph−0.1 mph

Each size on its own

Questions

Which is bigger, 315/55R20 or 355/45R21?
315/55R20 is taller by 0.06 inches (33.64" vs 33.58"). 355/45R21 is wider by 1.57 inches.
Can I replace 315/55R20 with 355/45R21?
Usually, yes. The diameter difference is −0.2%, 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 355/45R21 change my speedometer?
When your speedometer reads 60 mph with 355/45R21 fitted (calibrated for 315/55R20), your real speed is 59.9 mph. Your odometer will over-count miles by about 0.2%.
Will 355/45R21 lift my vehicle?
Ride height changes by half the diameter difference: −0.03 inches of ground clearance at the axle.

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