Tire Size Box

235/55R18 vs 235/65R18

235/65R18 compared with 235/55R18

+6.6% diameter, 0.0" width

235/65R18 is 30.03" tall vs 28.18". More than the 3% rule of thumb, so check carefully before switching.

28.2"18" rim5.1"9.3" wide
235/55R18 (solid) compared with 235/65R18 (dashed outline), drawn to scale

With 235/65R18 tires, when your speedometer says 60 mph you are really going

63.9 mph

235/65R18 is +1.85 inches (+6.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

235/55R18235/65R18Difference%
Overall diameter28.18"30.03"+1.85"+6.6%
Section width9.25"9.25"0.00"0.0%
Sidewall height5.09"6.01"+0.93"+18.2%
Circumference88.52"94.33"+5.81"+6.6%
Revolutions per mile715.8671.7−44.1−6.2%

Speedometer difference

If your vehicle came with 235/55R18 and you fit 235/65R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.3 mph+1.3 mph
30 mph32.0 mph+2.0 mph
40 mph42.6 mph+2.6 mph
50 mph53.3 mph+3.3 mph
60 mph63.9 mph+3.9 mph
65 mph69.3 mph+4.3 mph
70 mph74.6 mph+4.6 mph
75 mph79.9 mph+4.9 mph
80 mph85.3 mph+5.3 mph

Each size on its own

Questions

Which is bigger, 235/55R18 or 235/65R18?
235/65R18 is taller by 1.85 inches (30.03" vs 28.18"). 235/55R18 is the same width.
Can I replace 235/55R18 with 235/65R18?
It is not recommended without checking carefully. The diameter difference is +6.6%, 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 235/65R18 change my speedometer?
When your speedometer reads 60 mph with 235/65R18 fitted (calibrated for 235/55R18), your real speed is 63.9 mph. Your odometer will under-count miles by about 6.6%.
Will 235/65R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.93 inches of ground clearance at the axle.

Other comparisons