TireSizesFinder

315/35R21 vs 255/45R21

255/45R21 is 0.35 in (9.0 mm) taller than 315/35R21, a +1.19% diameter difference. With the speedometer set for 315/35R21, an indicated 60 mph is 60.7 mph on 255/45R21.

Is this a safe swap?

Diameter difference +1.19%: within the ±3% guideline

By overall diameter alone, 255/45R21 falls inside the ±3% range commonly used as a guideline when replacing 315/35R21. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 315/35R21 (29.68 in) vs 255/45R21 (30.04 in)315/35R2129.68 in × 12.40 in255/45R2130.04 in × 10.04 in
Side profile and tread width, drawn to the same scale.
315/35R21 vs 255/45R21 differences
Measurement315/35R21255/45R21Difference% diff
Overall diameter29.68 in (753.9 mm)30.04 in (762.9 mm)+0.35 in (+9.0 mm)+1.19%
Sidewall height4.34 in (110.3 mm)4.52 in (114.8 mm)+0.18 in (+4.5 mm)+4.08%
Section width12.40 in (315.0 mm)10.04 in (255.0 mm)−2.36 in (−60.0 mm)−19.05%
Circumference93.25 in (2,368.4 mm)94.36 in (2,396.7 mm)+1.11 in (+28.3 mm)+1.19%
Revolutions per mile679671−8−1.18%
True speed with the speedometer calibrated for 315/35R21
IndicatedActual on 315/35R21Actual on 255/45R21Difference
30 mph30.0 mph30.4 mph+0.4 mph
60 mph60.0 mph60.7 mph+0.7 mph
75 mph75.0 mph75.9 mph+0.9 mph

Ride-height change: 255/45R21 raises the vehicle by 0.18 in (4.5 mm) at the axle (half the +9.0 mm diameter difference).

255/45R21 is 0.35 in taller than 315/35R21 and 60 mm narrower, a 1.19% difference in overall diameter.

With the speedometer calibrated for 315/35R21, an indicated 60 mph is actually 60.7 mph on 255/45R21, and revolutions per mile change from 679 to 671.

Both sizes mount on 21-inch wheels.

Size pages

Other comparisons with 315/35R21

Other comparisons with 255/45R21

Source: Calculated from standard tire geometry formulas. Verified . How these numbers are calculated.