Tire Size Box

315/40R17 vs 315/45R17

315/45R17 compared with 315/40R17

+4.6% diameter, 0.0" width

315/45R17 is 28.16" tall vs 26.92". More than the 3% rule of thumb, so check carefully before switching.

26.9"17" rim5.0"12.4" wide
315/40R17 (solid) compared with 315/45R17 (dashed outline), drawn to scale

With 315/45R17 tires, when your speedometer says 60 mph you are really going

62.8 mph

315/45R17 is +1.24 inches (+4.6%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R17315/45R17Difference%
Overall diameter26.92"28.16"+1.24"+4.6%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.96"5.58"+0.62"+12.5%
Circumference84.58"88.47"+3.90"+4.6%
Revolutions per mile749.2716.2−33.0−4.4%

Speedometer difference

If your vehicle came with 315/40R17 and you fit 315/45R17, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.9 mph+0.9 mph
30 mph31.4 mph+1.4 mph
40 mph41.8 mph+1.8 mph
50 mph52.3 mph+2.3 mph
60 mph62.8 mph+2.8 mph
65 mph68.0 mph+3.0 mph
70 mph73.2 mph+3.2 mph
75 mph78.5 mph+3.5 mph
80 mph83.7 mph+3.7 mph

Each size on its own

Questions

Which is bigger, 315/40R17 or 315/45R17?
315/45R17 is taller by 1.24 inches (28.16" vs 26.92"). 315/40R17 is the same width.
Can I replace 315/40R17 with 315/45R17?
It is not recommended without checking carefully. The diameter difference is +4.6%, 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 315/45R17 change my speedometer?
When your speedometer reads 60 mph with 315/45R17 fitted (calibrated for 315/40R17), your real speed is 62.8 mph. Your odometer will under-count miles by about 4.6%.
Will 315/45R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

Other comparisons