Tire Size Box

315/40R18 vs 315/45R18

315/45R18 compared with 315/40R18

+4.4% diameter, 0.0" width

315/45R18 is 29.16" 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 315/45R18 (dashed outline), drawn to scale

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

62.7 mph

315/45R18 is +1.24 inches (+4.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R18315/45R18Difference%
Overall diameter27.92"29.16"+1.24"+4.4%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"5.58"+0.62"+12.5%
Circumference87.72"91.61"+3.90"+4.4%
Revolutions per mile722.3691.6−30.7−4.3%

Speedometer difference

If your vehicle came with 315/40R18 and you fit 315/45R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.3 mph+1.3 mph
40 mph41.8 mph+1.8 mph
50 mph52.2 mph+2.2 mph
60 mph62.7 mph+2.7 mph
65 mph67.9 mph+2.9 mph
70 mph73.1 mph+3.1 mph
75 mph78.3 mph+3.3 mph
80 mph83.6 mph+3.6 mph

Each size on its own

Questions

Which is bigger, 315/40R18 or 315/45R18?
315/45R18 is taller by 1.24 inches (29.16" vs 27.92"). 315/40R18 is the same width.
Can I replace 315/40R18 with 315/45R18?
It is not recommended without checking carefully. The diameter difference is +4.4%, 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 315/45R18 change my speedometer?
When your speedometer reads 60 mph with 315/45R18 fitted (calibrated for 315/40R18), your real speed is 62.7 mph. Your odometer will under-count miles by about 4.4%.
Will 315/45R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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