Tire Size Box

305/45R21 vs 305/55R21

305/55R21 compared with 305/45R21

+7.6% diameter, 0.0" width

305/55R21 is 34.21" tall vs 31.81". More than the 3% rule of thumb, so check carefully before switching.

31.8"21" rim5.4"12.0" wide
305/45R21 (solid) compared with 305/55R21 (dashed outline), drawn to scale

With 305/55R21 tires, when your speedometer says 60 mph you are really going

64.5 mph

305/55R21 is +2.40 inches (+7.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R21305/55R21Difference%
Overall diameter31.81"34.21"+2.40"+7.6%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.60"+1.20"+22.2%
Circumference99.92"107.47"+7.54"+7.6%
Revolutions per mile634.1589.6−44.5−7.0%

Speedometer difference

If your vehicle came with 305/45R21 and you fit 305/55R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.5 mph+1.5 mph
30 mph32.3 mph+2.3 mph
40 mph43.0 mph+3.0 mph
50 mph53.8 mph+3.8 mph
60 mph64.5 mph+4.5 mph
65 mph69.9 mph+4.9 mph
70 mph75.3 mph+5.3 mph
75 mph80.7 mph+5.7 mph
80 mph86.0 mph+6.0 mph

Each size on its own

Questions

Which is bigger, 305/45R21 or 305/55R21?
305/55R21 is taller by 2.40 inches (34.21" vs 31.81"). 305/45R21 is the same width.
Can I replace 305/45R21 with 305/55R21?
It is not recommended without checking carefully. The diameter difference is +7.6%, 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 305/55R21 change my speedometer?
When your speedometer reads 60 mph with 305/55R21 fitted (calibrated for 305/45R21), your real speed is 64.5 mph. Your odometer will under-count miles by about 7.6%.
Will 305/55R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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