Tire Size Box

325/40R18 vs 315/45R18

315/45R18 compared with 325/40R18

+3.3% diameter, −0.4" width

315/45R18 is 29.16" tall vs 28.24". More than the 3% rule of thumb, so check carefully before switching.

28.2"18" rim5.1"12.8" wide
325/40R18 (solid) compared with 315/45R18 (dashed outline), drawn to scale

With 315/45R18 tires, when your speedometer says 60 mph you are really going

62.0 mph

315/45R18 is +0.93 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/40R18315/45R18Difference%
Overall diameter28.24"29.16"+0.93"+3.3%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height5.12"5.58"+0.46"+9.0%
Circumference88.71"91.61"+2.91"+3.3%
Revolutions per mile714.3691.6−22.7−3.2%

Speedometer difference

If your vehicle came with 325/40R18 and you fit 315/45R18, 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 mph51.6 mph+1.6 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, 325/40R18 or 315/45R18?
315/45R18 is taller by 0.93 inches (29.16" vs 28.24"). 325/40R18 is wider by 0.39 inches.
Can I replace 325/40R18 with 315/45R18?
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 315/45R18 change my speedometer?
When your speedometer reads 60 mph with 315/45R18 fitted (calibrated for 325/40R18), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 315/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

Other comparisons