Tire Size Box

315/30R21 vs 305/35R21

305/35R21 compared with 315/30R21

+3.4% diameter, −0.4" width

305/35R21 is 29.41" 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 305/35R21 (dashed outline), drawn to scale

With 305/35R21 tires, when your speedometer says 60 mph you are really going

62.0 mph

305/35R21 is +0.96 inches (+3.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/30R21305/35R21Difference%
Overall diameter28.44"29.41"+0.96"+3.4%
Section width12.40"12.01"−0.39"−3.2%
Sidewall height3.72"4.20"+0.48"+13.0%
Circumference89.35"92.38"+3.03"+3.4%
Revolutions per mile709.1685.9−23.3−3.3%

Speedometer difference

If your vehicle came with 315/30R21 and you fit 305/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.0 mph+1.0 mph
40 mph41.4 mph+1.4 mph
50 mph51.7 mph+1.7 mph
60 mph62.0 mph+2.0 mph
65 mph67.2 mph+2.2 mph
70 mph72.4 mph+2.4 mph
75 mph77.5 mph+2.5 mph
80 mph82.7 mph+2.7 mph

Each size on its own

Questions

Which is bigger, 315/30R21 or 305/35R21?
305/35R21 is taller by 0.96 inches (29.41" vs 28.44"). 315/30R21 is wider by 0.39 inches.
Can I replace 315/30R21 with 305/35R21?
It is not recommended without checking carefully. The diameter difference is +3.4%, 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 305/35R21 change my speedometer?
When your speedometer reads 60 mph with 305/35R21 fitted (calibrated for 315/30R21), your real speed is 62.0 mph. Your odometer will under-count miles by about 3.4%.
Will 305/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.48 inches of ground clearance at the axle.

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