Tire Size Box

305/40R18 vs 315/40R18

315/40R18 compared with 305/40R18

+1.1% diameter, +0.4" width

315/40R18 is 27.92" tall vs 27.61". Within the 3% rule, so usually interchangeable.

27.6"18" rim4.8"12.0" wide
305/40R18 (solid) compared with 315/40R18 (dashed outline), drawn to scale

With 315/40R18 tires, when your speedometer says 60 mph you are really going

60.7 mph

315/40R18 is +0.31 inches (+1.1%) in overall diameter. That is within the 3% most shops and makers allow.

305/40R18315/40R18Difference%
Overall diameter27.61"27.92"+0.31"+1.1%
Section width12.01"12.40"+0.39"+3.3%
Sidewall height4.80"4.96"+0.16"+3.3%
Circumference86.73"87.72"+0.99"+1.1%
Revolutions per mile730.6722.3−8.2−1.1%

Speedometer difference

If your vehicle came with 305/40R18 and you fit 315/40R18, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.2 mph+0.2 mph
30 mph30.3 mph+0.3 mph
40 mph40.5 mph+0.5 mph
50 mph50.6 mph+0.6 mph
60 mph60.7 mph+0.7 mph
65 mph65.7 mph+0.7 mph
70 mph70.8 mph+0.8 mph
75 mph75.9 mph+0.9 mph
80 mph80.9 mph+0.9 mph

Each size on its own

Questions

Which is bigger, 305/40R18 or 315/40R18?
315/40R18 is taller by 0.31 inches (27.92" vs 27.61"). 315/40R18 is wider by 0.39 inches.
Can I replace 305/40R18 with 315/40R18?
Usually, yes. The diameter difference is +1.1%, inside the 3% rule of thumb most tire shops use. Check that the wider or taller tire clears your suspension and fenders, and that its load index meets your placard.
How much will 315/40R18 change my speedometer?
When your speedometer reads 60 mph with 315/40R18 fitted (calibrated for 305/40R18), your real speed is 60.7 mph. Your odometer will under-count miles by about 1.1%.
Will 315/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.16 inches of ground clearance at the axle.

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