Tire Size Box

325/30R21 vs 315/35R21

315/35R21 compared with 325/30R21

+3.5% diameter, −0.4" width

315/35R21 is 29.68" tall vs 28.68". More than the 3% rule of thumb, so check carefully before switching.

28.7"21" rim3.8"12.8" wide
325/30R21 (solid) compared with 315/35R21 (dashed outline), drawn to scale

With 315/35R21 tires, when your speedometer says 60 mph you are really going

62.1 mph

315/35R21 is +1.00 inches (+3.5%) in overall diameter. That is more than the 3% most shops and makers recommend.

325/30R21315/35R21Difference%
Overall diameter28.68"29.68"+1.00"+3.5%
Section width12.80"12.40"−0.39"−3.1%
Sidewall height3.84"4.34"+0.50"+13.1%
Circumference90.09"93.25"+3.15"+3.5%
Revolutions per mile703.3679.5−23.8−3.4%

Speedometer difference

If your vehicle came with 325/30R21 and you fit 315/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph20.7 mph+0.7 mph
30 mph31.1 mph+1.1 mph
40 mph41.4 mph+1.4 mph
50 mph51.8 mph+1.8 mph
60 mph62.1 mph+2.1 mph
65 mph67.3 mph+2.3 mph
70 mph72.5 mph+2.5 mph
75 mph77.6 mph+2.6 mph
80 mph82.8 mph+2.8 mph

Each size on its own

Questions

Which is bigger, 325/30R21 or 315/35R21?
315/35R21 is taller by 1.00 inches (29.68" vs 28.68"). 325/30R21 is wider by 0.39 inches.
Can I replace 325/30R21 with 315/35R21?
It is not recommended without checking carefully. The diameter difference is +3.5%, 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 315/35R21 change my speedometer?
When your speedometer reads 60 mph with 315/35R21 fitted (calibrated for 325/30R21), your real speed is 62.1 mph. Your odometer will under-count miles by about 3.5%.
Will 315/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +0.50 inches of ground clearance at the axle.

Other comparisons