Tire Size Box

315/40R19 vs 305/45R19

305/45R19 compared with 315/40R19

+3.1% diameter, −0.4" width

305/45R19 is 29.81" tall vs 28.92". More than the 3% rule of thumb, so check carefully before switching.

28.9"19" rim5.0"12.4" wide
315/40R19 (solid) compared with 305/45R19 (dashed outline), drawn to scale

With 305/45R19 tires, when your speedometer says 60 mph you are really going

61.8 mph

305/45R19 is +0.89 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/40R19305/45R19Difference%
Overall diameter28.92"29.81"+0.89"+3.1%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.96"5.40"+0.44"+8.9%
Circumference90.86"93.64"+2.78"+3.1%
Revolutions per mile697.3676.6−20.7−3.0%

Speedometer difference

If your vehicle came with 315/40R19 and you fit 305/45R19, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.6 mph+0.6 mph
30 mph30.9 mph+0.9 mph
40 mph41.2 mph+1.2 mph
50 mph51.5 mph+1.5 mph
60 mph61.8 mph+1.8 mph
65 mph67.0 mph+2.0 mph
70 mph72.1 mph+2.1 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 315/40R19 or 305/45R19?
305/45R19 is taller by 0.89 inches (29.81" vs 28.92"). 315/40R19 is wider by 0.39 inches.
Can I replace 315/40R19 with 305/45R19?
It is not recommended without checking carefully. The diameter difference is +3.1%, 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 305/45R19 change my speedometer?
When your speedometer reads 60 mph with 305/45R19 fitted (calibrated for 315/40R19), your real speed is 61.8 mph. Your odometer will under-count miles by about 3.1%.
Will 305/45R19 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

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