Tire Size Box

305/55R20 vs 305/65R20

305/65R20 compared with 305/55R20

+7.2% diameter, 0.0" width

305/65R20 is 35.61" tall vs 33.21". More than the 3% rule of thumb, so check carefully before switching.

33.2"20" rim6.6"12.0" wide
305/55R20 (solid) compared with 305/65R20 (dashed outline), drawn to scale

With 305/65R20 tires, when your speedometer says 60 mph you are really going

64.3 mph

305/65R20 is +2.40 inches (+7.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/55R20305/65R20Difference%
Overall diameter33.21"35.61"+2.40"+7.2%
Section width12.01"12.01"0.00"0.0%
Sidewall height6.60"7.81"+1.20"+18.2%
Circumference104.33"111.87"+7.54"+7.2%
Revolutions per mile607.3566.4−41.0−6.7%

Speedometer difference

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

Speedometer showsActual speedDifference
20 mph21.4 mph+1.4 mph
30 mph32.2 mph+2.2 mph
40 mph42.9 mph+2.9 mph
50 mph53.6 mph+3.6 mph
60 mph64.3 mph+4.3 mph
65 mph69.7 mph+4.7 mph
70 mph75.1 mph+5.1 mph
75 mph80.4 mph+5.4 mph
80 mph85.8 mph+5.8 mph

Each size on its own

Questions

Which is bigger, 305/55R20 or 305/65R20?
305/65R20 is taller by 2.40 inches (35.61" vs 33.21"). 305/55R20 is the same width.
Can I replace 305/55R20 with 305/65R20?
It is not recommended without checking carefully. The diameter difference is +7.2%, 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 305/65R20 change my speedometer?
When your speedometer reads 60 mph with 305/65R20 fitted (calibrated for 305/55R20), your real speed is 64.3 mph. Your odometer will under-count miles by about 7.2%.
Will 305/65R20 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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