Tire Size Box

185/75R16 vs 315/40R17

315/40R17 compared with 185/75R16

0.0% diameter, +5.1" width

315/40R17 is 26.92" tall vs 26.93". Within the 3% rule, so usually interchangeable.

26.9"16" rim5.5"7.3" wide
185/75R16 (solid) compared with 315/40R17 (dashed outline), drawn to scale

With 315/40R17 tires, when your speedometer says 60 mph you are really going

60.0 mph

315/40R17 is 0.00 inches (0.0%) in overall diameter. That is within the 3% most shops and makers allow.

185/75R16315/40R17Difference%
Overall diameter26.93"26.92"0.00"0.0%
Section width7.28"12.40"+5.12"+70.3%
Sidewall height5.46"4.96"−0.50"−9.2%
Circumference84.59"84.58"−0.01"0.0%
Revolutions per mile749.0749.2+0.10.0%

Speedometer difference

If your vehicle came with 185/75R16 and you fit 315/40R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.0 mph0.0 mph
30 mph30.0 mph0.0 mph
40 mph40.0 mph0.0 mph
50 mph50.0 mph0.0 mph
60 mph60.0 mph0.0 mph
65 mph65.0 mph0.0 mph
70 mph70.0 mph0.0 mph
75 mph75.0 mph0.0 mph
80 mph80.0 mph0.0 mph

Each size on its own

Questions

Which is bigger, 185/75R16 or 315/40R17?
They are the same overall diameter, 26.93 inches.
Can I replace 185/75R16 with 315/40R17?
Usually, yes. The diameter difference is 0.0%, 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 315/40R17 change my speedometer?
When your speedometer reads 60 mph with 315/40R17 fitted (calibrated for 185/75R16), your real speed is 60.0 mph. Your odometer will over-count miles by about 0.0%.
Will 315/40R17 lift my vehicle?
Ride height changes by half the diameter difference: 0.00 inches of ground clearance at the axle.

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