Tire Size Box

315/35R18 vs 305/40R18

305/40R18 compared with 315/35R18

+3.5% diameter, −0.4" width

305/40R18 is 27.61" tall vs 26.68". More than the 3% rule of thumb, so check carefully before switching.

26.7"18" rim4.3"12.4" wide
315/35R18 (solid) compared with 305/40R18 (dashed outline), drawn to scale

With 305/40R18 tires, when your speedometer says 60 mph you are really going

62.1 mph

305/40R18 is +0.93 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R18305/40R18Difference%
Overall diameter26.68"27.61"+0.93"+3.5%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.34"4.80"+0.46"+10.7%
Circumference83.82"86.73"+2.91"+3.5%
Revolutions per mile755.9730.6−25.3−3.4%

Speedometer difference

If your vehicle came with 315/35R18 and you fit 305/40R18, 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 mph67.3 mph+2.3 mph
70 mph72.4 mph+2.4 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 315/35R18 or 305/40R18?
305/40R18 is taller by 0.93 inches (27.61" vs 26.68"). 315/35R18 is wider by 0.39 inches.
Can I replace 315/35R18 with 305/40R18?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 305/40R18 change my speedometer?
When your speedometer reads 60 mph with 305/40R18 fitted (calibrated for 315/35R18), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 305/40R18 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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