Tire Size Box

315/25R21 vs 315/35R21

315/35R21 compared with 315/25R21

+9.1% diameter, 0.0" width

315/35R21 is 29.68" tall vs 27.20". More than the 3% rule of thumb, so check carefully before switching.

27.2"21" rim3.1"12.4" wide
315/25R21 (solid) compared with 315/35R21 (dashed outline), drawn to scale

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

65.5 mph

315/35R21 is +2.48 inches (+9.1%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/25R21315/35R21Difference%
Overall diameter27.20"29.68"+2.48"+9.1%
Section width12.40"12.40"0.00"0.0%
Sidewall height3.10"4.34"+1.24"+40.0%
Circumference85.45"93.25"+7.79"+9.1%
Revolutions per mile741.5679.5−62.0−8.4%

Speedometer difference

If your vehicle came with 315/25R21 and you fit 315/35R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.8 mph+1.8 mph
30 mph32.7 mph+2.7 mph
40 mph43.6 mph+3.6 mph
50 mph54.6 mph+4.6 mph
60 mph65.5 mph+5.5 mph
65 mph70.9 mph+5.9 mph
70 mph76.4 mph+6.4 mph
75 mph81.8 mph+6.8 mph
80 mph87.3 mph+7.3 mph

Each size on its own

Questions

Which is bigger, 315/25R21 or 315/35R21?
315/35R21 is taller by 2.48 inches (29.68" vs 27.20"). 315/25R21 is the same width.
Can I replace 315/25R21 with 315/35R21?
It is not recommended without checking carefully. The diameter difference is +9.1%, 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 315/35R21 change my speedometer?
When your speedometer reads 60 mph with 315/35R21 fitted (calibrated for 315/25R21), your real speed is 65.5 mph. Your odometer will under-count miles by about 9.1%.
Will 315/35R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

Other comparisons