Tire Size Box

225/40R18 vs 225/45R18

225/45R18 compared with 225/40R18

+3.5% diameter, 0.0" width

225/45R18 is 25.97" tall vs 25.09". More than the 3% rule of thumb, so check carefully before switching.

25.1"18" rim3.5"8.9" wide
225/40R18 (solid) compared with 225/45R18 (dashed outline), drawn to scale

With 225/45R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

225/45R18 is +0.89 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

225/40R18225/45R18Difference%
Overall diameter25.09"25.97"+0.89"+3.5%
Section width8.86"8.86"0.00"0.0%
Sidewall height3.54"3.99"+0.44"+12.5%
Circumference78.81"81.59"+2.78"+3.5%
Revolutions per mile803.9776.5−27.4−3.4%

Speedometer difference

If your vehicle came with 225/40R18 and you fit 225/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 225/40R18 or 225/45R18?
225/45R18 is taller by 0.89 inches (25.97" vs 25.09"). 225/40R18 is the same width.
Can I replace 225/40R18 with 225/45R18?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 225/45R18 change my speedometer?
When your speedometer reads 60 mph with 225/45R18 fitted (calibrated for 225/40R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 225/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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