Tire Size Box

315/35R21 vs 305/40R21

305/40R21 compared with 315/35R21

+3.1% diameter, −0.4" width

305/40R21 is 30.61" 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 305/40R21 (dashed outline), drawn to scale

With 305/40R21 tires, when your speedometer says 60 mph you are really going

61.9 mph

305/40R21 is +0.93 inches (+3.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R21305/40R21Difference%
Overall diameter29.68"30.61"+0.93"+3.1%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height4.34"4.80"+0.46"+10.7%
Circumference93.25"96.15"+2.91"+3.1%
Revolutions per mile679.5659.0−20.5−3.0%

Speedometer difference

If your vehicle came with 315/35R21 and you fit 305/40R21, 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 mph51.6 mph+1.6 mph
60 mph61.9 mph+1.9 mph
65 mph67.0 mph+2.0 mph
70 mph72.2 mph+2.2 mph
75 mph77.3 mph+2.3 mph
80 mph82.5 mph+2.5 mph

Each size on its own

Questions

Which is bigger, 315/35R21 or 305/40R21?
305/40R21 is taller by 0.93 inches (30.61" vs 29.68"). 315/35R21 is wider by 0.39 inches.
Can I replace 315/35R21 with 305/40R21?
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 305/40R21 change my speedometer?
When your speedometer reads 60 mph with 305/40R21 fitted (calibrated for 315/35R21), your real speed is 61.9 mph. Your odometer will under-count miles by about 3.1%.
Will 305/40R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.46 inches of ground clearance at the axle.

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