Tire Size Box

215/40R18 vs 215/45R18

215/45R18 compared with 215/40R18

+3.4% diameter, 0.0" width

215/45R18 is 25.62" tall vs 24.77". More than the 3% rule of thumb, so check carefully before switching.

24.8"18" rim3.4"8.5" wide
215/40R18 (solid) compared with 215/45R18 (dashed outline), drawn to scale

With 215/45R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

215/45R18 is +0.85 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

215/40R18215/45R18Difference%
Overall diameter24.77"25.62"+0.85"+3.4%
Section width8.46"8.46"0.00"0.0%
Sidewall height3.39"3.81"+0.42"+12.5%
Circumference77.82"80.48"+2.66"+3.4%
Revolutions per mile814.2787.3−26.9−3.3%

Speedometer difference

If your vehicle came with 215/40R18 and you fit 215/45R18, 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 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.1 mph+2.1 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 215/40R18 or 215/45R18?
215/45R18 is taller by 0.85 inches (25.62" vs 24.77"). 215/40R18 is the same width.
Can I replace 215/40R18 with 215/45R18?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 215/45R18 change my speedometer?
When your speedometer reads 60 mph with 215/45R18 fitted (calibrated for 215/40R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.4%.
Will 215/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.42 inches of ground clearance at the axle.

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