Tire Size Box

315/35R21 vs 315/40R21

315/40R21 compared with 315/35R21

+4.2% diameter, 0.0" width

315/40R21 is 30.92" tall vs 29.68". More than the 3% rule of thumb, so check carefully before switching.

29.7"21" rim4.3"12.4" wide
315/35R21 (solid) compared with 315/40R21 (dashed outline), drawn to scale

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

62.5 mph

315/40R21 is +1.24 inches (+4.2%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R21315/40R21Difference%
Overall diameter29.68"30.92"+1.24"+4.2%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.34"4.96"+0.62"+14.3%
Circumference93.25"97.14"+3.90"+4.2%
Revolutions per mile679.5652.2−27.3−4.0%

Speedometer difference

If your vehicle came with 315/35R21 and you fit 315/40R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.8 mph+0.8 mph
30 mph31.3 mph+1.3 mph
40 mph41.7 mph+1.7 mph
50 mph52.1 mph+2.1 mph
60 mph62.5 mph+2.5 mph
65 mph67.7 mph+2.7 mph
70 mph72.9 mph+2.9 mph
75 mph78.1 mph+3.1 mph
80 mph83.3 mph+3.3 mph

Each size on its own

Questions

Which is bigger, 315/35R21 or 315/40R21?
315/40R21 is taller by 1.24 inches (30.92" vs 29.68"). 315/35R21 is the same width.
Can I replace 315/35R21 with 315/40R21?
It is not recommended without checking carefully. The diameter difference is +4.2%, 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 315/35R21), your real speed is 62.5 mph. Your odometer will under-count miles by about 4.2%.
Will 315/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.62 inches of ground clearance at the axle.

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