Tire Size Box

325/65R19 vs 305/65R20

305/65R20 compared with 325/65R19

−0.1% diameter, −0.8" width

305/65R20 is 35.61" tall vs 35.63". Within the 3% rule, so usually interchangeable.

35.6"19" rim8.3"12.8" wide
325/65R19 (solid) compared with 305/65R20 (dashed outline), drawn to scale

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

60.0 mph

305/65R20 is −0.02 inches (−0.1%) in overall diameter. That is within the 3% most shops and makers allow.

325/65R19305/65R20Difference%
Overall diameter35.63"35.61"−0.02"−0.1%
Section width12.80"12.01"−0.79"−6.2%
Sidewall height8.32"7.81"−0.51"−6.2%
Circumference111.95"111.87"−0.07"−0.1%
Revolutions per mile566.0566.4+0.4+0.1%

Speedometer difference

If your vehicle came with 325/65R19 and you fit 305/65R20, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.0 mph0.0 mph
40 mph40.0 mph0.0 mph
50 mph50.0 mph0.0 mph
60 mph60.0 mph0.0 mph
65 mph65.0 mph0.0 mph
70 mph70.0 mph0.0 mph
75 mph75.0 mph0.0 mph
80 mph79.9 mph−0.1 mph

Each size on its own

Questions

Which is bigger, 325/65R19 or 305/65R20?
325/65R19 is taller by 0.02 inches (35.63" vs 35.61"). 325/65R19 is wider by 0.79 inches.
Can I replace 325/65R19 with 305/65R20?
Usually, yes. The diameter difference is −0.1%, 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 305/65R20 change my speedometer?
When your speedometer reads 60 mph with 305/65R20 fitted (calibrated for 325/65R19), your real speed is 60.0 mph. Your odometer will over-count miles by about 0.1%.
Will 305/65R20 lift my vehicle?
Ride height changes by half the diameter difference: −0.01 inches of ground clearance at the axle.

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