Tire Size Box

305/45R20 vs 305/55R20

305/55R20 compared with 305/45R20

+7.8% diameter, 0.0" width

305/55R20 is 33.21" tall vs 30.81". More than the 3% rule of thumb, so check carefully before switching.

30.8"20" rim5.4"12.0" wide
305/45R20 (solid) compared with 305/55R20 (dashed outline), drawn to scale

With 305/55R20 tires, when your speedometer says 60 mph you are really going

64.7 mph

305/55R20 is +2.40 inches (+7.8%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/45R20305/55R20Difference%
Overall diameter30.81"33.21"+2.40"+7.8%
Section width12.01"12.01"0.00"0.0%
Sidewall height5.40"6.60"+1.20"+22.2%
Circumference96.78"104.33"+7.54"+7.8%
Revolutions per mile654.7607.3−47.3−7.2%

Speedometer difference

If your vehicle came with 305/45R20 and you fit 305/55R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.6 mph+1.6 mph
30 mph32.3 mph+2.3 mph
40 mph43.1 mph+3.1 mph
50 mph53.9 mph+3.9 mph
60 mph64.7 mph+4.7 mph
65 mph70.1 mph+5.1 mph
70 mph75.5 mph+5.5 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 305/45R20 or 305/55R20?
305/55R20 is taller by 2.40 inches (33.21" vs 30.81"). 305/45R20 is the same width.
Can I replace 305/45R20 with 305/55R20?
It is not recommended without checking carefully. The diameter difference is +7.8%, 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 305/55R20 change my speedometer?
When your speedometer reads 60 mph with 305/55R20 fitted (calibrated for 305/45R20), your real speed is 64.7 mph. Your odometer will under-count miles by about 7.8%.
Will 305/55R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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