Tire Size Box

315/35R21 vs 315/45R21

315/45R21 compared with 315/35R21

+8.4% diameter, 0.0" width

315/45R21 is 32.16" 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 315/45R21 (dashed outline), drawn to scale

With 315/45R21 tires, when your speedometer says 60 mph you are really going

65.0 mph

315/45R21 is +2.48 inches (+8.4%) in overall diameter. That is more than the 3% most shops and makers recommend.

315/35R21315/45R21Difference%
Overall diameter29.68"32.16"+2.48"+8.4%
Section width12.40"12.40"0.00"0.0%
Sidewall height4.34"5.58"+1.24"+28.6%
Circumference93.25"101.04"+7.79"+8.4%
Revolutions per mile679.5627.1−52.4−7.7%

Speedometer difference

If your vehicle came with 315/35R21 and you fit 315/45R21, here is your real speed for each speedometer reading:

Speedometer showsActual speedDifference
20 mph21.7 mph+1.7 mph
30 mph32.5 mph+2.5 mph
40 mph43.3 mph+3.3 mph
50 mph54.2 mph+4.2 mph
60 mph65.0 mph+5.0 mph
65 mph70.4 mph+5.4 mph
70 mph75.8 mph+5.8 mph
75 mph81.3 mph+6.3 mph
80 mph86.7 mph+6.7 mph

Each size on its own

Questions

Which is bigger, 315/35R21 or 315/45R21?
315/45R21 is taller by 2.48 inches (32.16" vs 29.68"). 315/35R21 is the same width.
Can I replace 315/35R21 with 315/45R21?
It is not recommended without checking carefully. The diameter difference is +8.4%, 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 315/45R21 change my speedometer?
When your speedometer reads 60 mph with 315/45R21 fitted (calibrated for 315/35R21), your real speed is 65.0 mph. Your odometer will under-count miles by about 8.4%.
Will 315/45R21 lift my vehicle?
Ride height changes by half the diameter difference: +1.24 inches of ground clearance at the axle.

Other comparisons