Tire Size Box

315/35R17 vs 305/40R17

305/40R17 compared with 315/35R17

+3.6% diameter, −0.4" width

305/40R17 is 26.61" tall vs 25.68". More than the 3% rule of thumb, so check carefully before switching.

25.7"17" rim4.3"12.4" wide
315/35R17 (solid) compared with 305/40R17 (dashed outline), drawn to scale

With 305/40R17 tires, when your speedometer says 60 mph you are really going

62.2 mph

305/40R17 is +0.93 inches (+3.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R17305/40R17Difference%
Overall diameter25.68"26.61"+0.93"+3.6%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.34"4.80"+0.46"+10.7%
Circumference80.68"83.59"+2.91"+3.6%
Revolutions per mile785.3758.0−27.3−3.5%

Speedometer difference

If your vehicle came with 315/35R17 and you fit 305/40R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.2 mph+2.2 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.7 mph+2.7 mph
80 mph82.9 mph+2.9 mph

Each size on its own

Questions

Which is bigger, 315/35R17 or 305/40R17?
305/40R17 is taller by 0.93 inches (26.61" vs 25.68"). 315/35R17 is wider by 0.39 inches.
Can I replace 315/35R17 with 305/40R17?
It is not recommended without checking carefully. The diameter difference is +3.6%, 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 305/40R17 change my speedometer?
When your speedometer reads 60 mph with 305/40R17 fitted (calibrated for 315/35R17), your real speed is 62.2 mph. Your odometer will under-count miles by about 3.6%.
Will 305/40R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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