Tire Size Box

205/45R18 vs 205/50R18

205/50R18 compared with 205/45R18

+3.2% diameter, 0.0" width

205/50R18 is 26.07" tall vs 25.26". More than the 3% rule of thumb, so check carefully before switching.

25.3"18" rim3.6"8.1" wide
205/45R18 (solid) compared with 205/50R18 (dashed outline), drawn to scale

With 205/50R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

205/50R18 is +0.81 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

205/45R18205/50R18Difference%
Overall diameter25.26"26.07"+0.81"+3.2%
Section width8.07"8.07"0.00"0.0%
Sidewall height3.63"4.04"+0.40"+11.1%
Circumference79.37"81.90"+2.54"+3.2%
Revolutions per mile798.3773.6−24.7−3.1%

Speedometer difference

If your vehicle came with 205/45R18 and you fit 205/50R18, 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, 205/45R18 or 205/50R18?
205/50R18 is taller by 0.81 inches (26.07" vs 25.26"). 205/45R18 is the same width.
Can I replace 205/45R18 with 205/50R18?
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 205/50R18 change my speedometer?
When your speedometer reads 60 mph with 205/50R18 fitted (calibrated for 205/45R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 205/50R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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