Tire Size Box

315/30R21 vs 315/40R21

315/40R21 compared with 315/30R21

+8.7% diameter, 0.0" width

315/40R21 is 30.92" tall vs 28.44". More than the 3% rule of thumb, so check carefully before switching.

28.4"21" rim3.7"12.4" wide
315/30R21 (solid) compared with 315/40R21 (dashed outline), drawn to scale

With 315/40R21 tires, when your speedometer says 60 mph you are really going

65.2 mph

315/40R21 is +2.48 inches (+8.7%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/30R21315/40R21Difference%
Overall diameter28.44"30.92"+2.48"+8.7%
Section width12.40"12.40"0.00"0.0%
Sidewall height3.72"4.96"+1.24"+33.3%
Circumference89.35"97.14"+7.79"+8.7%
Revolutions per mile709.1652.2−56.9−8.0%

Speedometer difference

If your vehicle came with 315/30R21 and you fit 315/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.6 mph+2.6 mph
40 mph43.5 mph+3.5 mph
50 mph54.4 mph+4.4 mph
60 mph65.2 mph+5.2 mph
65 mph70.7 mph+5.7 mph
70 mph76.1 mph+6.1 mph
75 mph81.5 mph+6.5 mph
80 mph87.0 mph+7.0 mph

Each size on its own

Questions

Which is bigger, 315/30R21 or 315/40R21?
315/40R21 is taller by 2.48 inches (30.92" vs 28.44"). 315/30R21 is the same width.
Can I replace 315/30R21 with 315/40R21?
It is not recommended without checking carefully. The diameter difference is +8.7%, 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 315/40R21 change my speedometer?
When your speedometer reads 60 mph with 315/40R21 fitted (calibrated for 315/30R21), your real speed is 65.2 mph. Your odometer will under-count miles by about 8.7%.
Will 315/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

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