Tire Size Box

315/40R18 vs 305/45R18

305/45R18 compared with 315/40R18

+3.2% diameter, −0.4" width

305/45R18 is 28.81" tall vs 27.92". More than the 3% rule of thumb, so check carefully before switching.

27.9"18" rim5.0"12.4" wide
315/40R18 (solid) compared with 305/45R18 (dashed outline), drawn to scale

With 305/45R18 tires, when your speedometer says 60 mph you are really going

61.9 mph

305/45R18 is +0.89 inches (+3.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R18305/45R18Difference%
Overall diameter27.92"28.81"+0.89"+3.2%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.96"5.40"+0.44"+8.9%
Circumference87.72"90.50"+2.78"+3.2%
Revolutions per mile722.3700.1−22.2−3.1%

Speedometer difference

If your vehicle came with 315/40R18 and you fit 305/45R18, 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 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 315/40R18 or 305/45R18?
305/45R18 is taller by 0.89 inches (28.81" vs 27.92"). 315/40R18 is wider by 0.39 inches.
Can I replace 315/40R18 with 305/45R18?
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 305/45R18 change my speedometer?
When your speedometer reads 60 mph with 305/45R18 fitted (calibrated for 315/40R18), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.2%.
Will 305/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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