Tire Size Box

305/55R18 vs 305/65R18

305/65R18 compared with 305/55R18

+7.7% diameter, 0.0" width

305/65R18 is 33.61" tall vs 31.21". More than the 3% rule of thumb, so check carefully before switching.

31.2"18" rim6.6"12.0" wide
305/55R18 (solid) compared with 305/65R18 (dashed outline), drawn to scale

With 305/65R18 tires, when your speedometer says 60 mph you are really going

64.6 mph

305/65R18 is +2.40 inches (+7.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

305/55R18305/65R18Difference%
Overall diameter31.21"33.61"+2.40"+7.7%
Section width12.01"12.01"0.00"0.0%
Sidewall height6.60"7.81"+1.20"+18.2%
Circumference98.04"105.59"+7.54"+7.7%
Revolutions per mile646.2600.1−46.2−7.1%

Speedometer difference

If your vehicle came with 305/55R18 and you fit 305/65R18, 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 mph43.1 mph+3.1 mph
50 mph53.8 mph+3.8 mph
60 mph64.6 mph+4.6 mph
65 mph70.0 mph+5.0 mph
70 mph75.4 mph+5.4 mph
75 mph80.8 mph+5.8 mph
80 mph86.2 mph+6.2 mph

Each size on its own

Questions

Which is bigger, 305/55R18 or 305/65R18?
305/65R18 is taller by 2.40 inches (33.61" vs 31.21"). 305/55R18 is the same width.
Can I replace 305/55R18 with 305/65R18?
It is not recommended without checking carefully. The diameter difference is +7.7%, 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 305/65R18 change my speedometer?
When your speedometer reads 60 mph with 305/65R18 fitted (calibrated for 305/55R18), your real speed is 64.6 mph. Your odometer will under-count miles by about 7.7%.
Will 305/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +1.20 inches of ground clearance at the axle.

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