Tire Size Box

205/40R18 vs 205/45R18

205/45R18 compared with 205/40R18

+3.3% diameter, 0.0" width

205/45R18 is 25.26" tall vs 24.46". More than the 3% rule of thumb, so check carefully before switching.

24.5"18" rim3.2"8.1" wide
205/40R18 (solid) compared with 205/45R18 (dashed outline), drawn to scale

With 205/45R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

205/45R18 is +0.81 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

205/40R18205/45R18Difference%
Overall diameter24.46"25.26"+0.81"+3.3%
Section width8.07"8.07"0.00"0.0%
Sidewall height3.23"3.63"+0.40"+12.5%
Circumference76.83"79.37"+2.54"+3.3%
Revolutions per mile824.6798.3−26.3−3.2%

Speedometer difference

If your vehicle came with 205/40R18 and you fit 205/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.3 mph+1.3 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 205/40R18 or 205/45R18?
205/45R18 is taller by 0.81 inches (25.26" vs 24.46"). 205/40R18 is the same width.
Can I replace 205/40R18 with 205/45R18?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 205/45R18 change my speedometer?
When your speedometer reads 60 mph with 205/45R18 fitted (calibrated for 205/40R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 205/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.40 inches of ground clearance at the axle.

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