Tire Size Box

315/40R17 vs 305/45R17

305/45R17 compared with 315/40R17

+3.3% diameter, −0.4" width

305/45R17 is 27.81" 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 305/45R17 (dashed outline), drawn to scale

With 305/45R17 tires, when your speedometer says 60 mph you are really going

62.0 mph

305/45R17 is +0.89 inches (+3.3%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R17305/45R17Difference%
Overall diameter26.92"27.81"+0.89"+3.3%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.96"5.40"+0.44"+8.9%
Circumference84.58"87.36"+2.78"+3.3%
Revolutions per mile749.2725.3−23.9−3.2%

Speedometer difference

If your vehicle came with 315/40R17 and you fit 305/45R17, 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 mph41.3 mph+1.3 mph
50 mph51.6 mph+1.6 mph
60 mph62.0 mph+2.0 mph
65 mph67.1 mph+2.1 mph
70 mph72.3 mph+2.3 mph
75 mph77.5 mph+2.5 mph
80 mph82.6 mph+2.6 mph

Each size on its own

Questions

Which is bigger, 315/40R17 or 305/45R17?
305/45R17 is taller by 0.89 inches (27.81" vs 26.92"). 315/40R17 is wider by 0.39 inches.
Can I replace 315/40R17 with 305/45R17?
It is not recommended without checking carefully. The diameter difference is +3.3%, 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 305/45R17 change my speedometer?
When your speedometer reads 60 mph with 305/45R17 fitted (calibrated for 315/40R17), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.3%.
Will 305/45R17 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

Other comparisons