Tire Size Box

185/40R17 vs 185/45R17

185/45R17 compared with 185/40R17

+3.2% diameter, 0.0" width

185/45R17 is 23.56" tall vs 22.83". More than the 3% rule of thumb, so check carefully before switching.

22.8"17" rim2.9"7.3" wide
185/40R17 (solid) compared with 185/45R17 (dashed outline), drawn to scale

With 185/45R17 tires, when your speedometer says 60 mph you are really going

61.9 mph

185/45R17 is +0.73 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

185/40R17185/45R17Difference%
Overall diameter22.83"23.56"+0.73"+3.2%
Section width7.28"7.28"0.00"0.0%
Sidewall height2.91"3.28"+0.36"+12.5%
Circumference71.71"74.00"+2.29"+3.2%
Revolutions per mile883.5856.2−27.3−3.1%

Speedometer difference

If your vehicle came with 185/40R17 and you fit 185/45R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.1 mph+2.1 mph
70 mph72.2 mph+2.2 mph
75 mph77.4 mph+2.4 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 185/40R17 or 185/45R17?
185/45R17 is taller by 0.73 inches (23.56" vs 22.83"). 185/40R17 is the same width.
Can I replace 185/40R17 with 185/45R17?
It is not recommended without checking carefully. The diameter difference is +3.2%, 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 185/45R17 change my speedometer?
When your speedometer reads 60 mph with 185/45R17 fitted (calibrated for 185/40R17), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 185/45R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.36 inches of ground clearance at the axle.

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