Tire Size Box

305/55R17 vs 205/75R18

205/75R18 compared with 305/55R17

−0.3% diameter, −3.9" width

205/75R18 is 30.11" tall vs 30.21". Within the 3% rule, so usually interchangeable.

30.2"17" rim6.6"12.0" wide
305/55R17 (solid) compared with 205/75R18 (dashed outline), drawn to scale

With 205/75R18 tires, when your speedometer says 60 mph you are really going

59.8 mph

205/75R18 is −0.10 inches (−0.3%) in overall diameter. That is within the 3% most shops and makers allow.

305/55R17205/75R18Difference%
Overall diameter30.21"30.11"−0.10"−0.3%
Section width12.01"8.07"−3.94"−32.8%
Sidewall height6.60"6.05"−0.55"−8.3%
Circumference94.90"94.58"−0.32"−0.3%
Revolutions per mile667.6669.9+2.3+0.3%

Speedometer difference

If your vehicle came with 305/55R17 and you fit 205/75R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph19.9 mph−0.1 mph
30 mph29.9 mph−0.1 mph
40 mph39.9 mph−0.1 mph
50 mph49.8 mph−0.2 mph
60 mph59.8 mph−0.2 mph
65 mph64.8 mph−0.2 mph
70 mph69.8 mph−0.2 mph
75 mph74.7 mph−0.3 mph
80 mph79.7 mph−0.3 mph

Each size on its own

Questions

Which is bigger, 305/55R17 or 205/75R18?
305/55R17 is taller by 0.10 inches (30.21" vs 30.11"). 305/55R17 is wider by 3.94 inches.
Can I replace 305/55R17 with 205/75R18?
Usually, yes. The diameter difference is −0.3%, 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 205/75R18 change my speedometer?
When your speedometer reads 60 mph with 205/75R18 fitted (calibrated for 305/55R17), your real speed is 59.8 mph. Your odometer will over-count miles by about 0.3%.
Will 205/75R18 lift my vehicle?
Ride height changes by half the diameter difference: −0.05 inches of ground clearance at the axle.

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