Tire Size Box

255/45R15 vs 255/55R15

255/55R15 compared with 255/45R15

+8.4% diameter, 0.0" width

255/55R15 is 26.04" tall vs 24.04". More than the 3% rule of thumb, so check carefully before switching.

24.0"15" rim4.5"10.0" wide
255/45R15 (solid) compared with 255/55R15 (dashed outline), drawn to scale

With 255/55R15 tires, when your speedometer says 60 mph you are really going

65.0 mph

255/55R15 is +2.01 inches (+8.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

255/45R15255/55R15Difference%
Overall diameter24.04"26.04"+2.01"+8.4%
Section width10.04"10.04"0.00"0.0%
Sidewall height4.52"5.52"+1.00"+22.2%
Circumference75.51"81.82"+6.31"+8.4%
Revolutions per mile839.1774.4−64.7−7.7%

Speedometer difference

If your vehicle came with 255/45R15 and you fit 255/55R15, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.3 mph+6.3 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 255/45R15 or 255/55R15?
255/55R15 is taller by 2.01 inches (26.04" vs 24.04"). 255/45R15 is the same width.
Can I replace 255/45R15 with 255/55R15?
It is not recommended without checking carefully. The diameter difference is +8.4%, 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 255/55R15 change my speedometer?
When your speedometer reads 60 mph with 255/55R15 fitted (calibrated for 255/45R15), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.4%.
Will 255/55R15 lift my vehicle?
Ride height changes by half the diameter difference: +1.00 inches of ground clearance at the axle.

Other comparisons