Tire Size Box

185/45R17 vs 185/55R17

185/55R17 compared with 185/45R17

+6.2% diameter, 0.0" width

185/55R17 is 25.01" tall vs 23.56". More than the 3% rule of thumb, so check carefully before switching.

23.6"17" rim3.3"7.3" wide
185/45R17 (solid) compared with 185/55R17 (dashed outline), drawn to scale

With 185/55R17 tires, when your speedometer says 60 mph you are really going

63.7 mph

185/55R17 is +1.46 inches (+6.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

185/45R17185/55R17Difference%
Overall diameter23.56"25.01"+1.46"+6.2%
Section width7.28"7.28"0.00"0.0%
Sidewall height3.28"4.01"+0.73"+22.2%
Circumference74.00"78.58"+4.58"+6.2%
Revolutions per mile856.2806.3−49.9−5.8%

Speedometer difference

If your vehicle came with 185/45R17 and you fit 185/55R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.2 mph+1.2 mph
30 mph31.9 mph+1.9 mph
40 mph42.5 mph+2.5 mph
50 mph53.1 mph+3.1 mph
60 mph63.7 mph+3.7 mph
65 mph69.0 mph+4.0 mph
70 mph74.3 mph+4.3 mph
75 mph79.6 mph+4.6 mph
80 mph84.9 mph+4.9 mph

Each size on its own

Questions

Which is bigger, 185/45R17 or 185/55R17?
185/55R17 is taller by 1.46 inches (25.01" vs 23.56"). 185/45R17 is the same width.
Can I replace 185/45R17 with 185/55R17?
It is not recommended without checking carefully. The diameter difference is +6.2%, 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 185/55R17 change my speedometer?
When your speedometer reads 60 mph with 185/55R17 fitted (calibrated for 185/45R17), your real speed is 63.7 mph. Your odometer will under-count miles by about 6.2%.
Will 185/55R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.73 inches of ground clearance at the axle.

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