Tire Size Box

315/40R21 vs 305/45R21

305/45R21 compared with 315/40R21

+2.9% diameter, −0.4" width

305/45R21 is 31.81" tall vs 30.92". Within the 3% rule, so usually interchangeable.

30.9"21" rim5.0"12.4" wide
315/40R21 (solid) compared with 305/45R21 (dashed outline), drawn to scale

With 305/45R21 tires, when your speedometer says 60 mph you are really going

61.7 mph

305/45R21 is +0.89 inches (+2.9%) in overall diameter. That is within the 3% most shops and makers allow.

315/40R21305/45R21Difference%
Overall diameter30.92"31.81"+0.89"+2.9%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.96"5.40"+0.44"+8.9%
Circumference97.14"99.92"+2.78"+2.9%
Revolutions per mile652.2634.1−18.2−2.8%

Speedometer difference

If your vehicle came with 315/40R21 and you fit 305/45R21, 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 mph41.1 mph+1.1 mph
50 mph51.4 mph+1.4 mph
60 mph61.7 mph+1.7 mph
65 mph66.9 mph+1.9 mph
70 mph72.0 mph+2.0 mph
75 mph77.1 mph+2.1 mph
80 mph82.3 mph+2.3 mph

Each size on its own

Questions

Which is bigger, 315/40R21 or 305/45R21?
305/45R21 is taller by 0.89 inches (31.81" vs 30.92"). 315/40R21 is wider by 0.39 inches.
Can I replace 315/40R21 with 305/45R21?
Usually, yes. The diameter difference is +2.9%, 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 305/45R21 change my speedometer?
When your speedometer reads 60 mph with 305/45R21 fitted (calibrated for 315/40R21), your real speed is 61.7 mph. Your odometer will under-count miles by about 2.9%.
Will 305/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.44 inches of ground clearance at the axle.

Other comparisons