TireSizesFinder

255/70R15 vs 30x9.50R15

30x9.50R15 is 0.94 in (24.0 mm) taller than 255/70R15, a +3.25% diameter difference. With the speedometer set for 255/70R15, an indicated 60 mph is 62.0 mph on 30x9.50R15.

Is this a safe swap?

Diameter difference +3.25%: outside the ±3% guideline

By overall diameter alone, 30x9.50R15 falls outside the ±3% range commonly used as a guideline when replacing 255/70R15; speedometer and ride-height changes are larger. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 255/70R15 (29.06 in) vs 30x9.50R15 (30.00 in)255/70R1529.06 in × 10.04 in30x9.50R1530.00 in × 9.50 in
Side profile and tread width, drawn to the same scale.
255/70R15 vs 30x9.50R15 differences
Measurement255/70R1530x9.50R15Difference% diff
Overall diameter29.06 in (738.0 mm)30.00 in (762.0 mm)+0.94 in (+24.0 mm)+3.25%
Sidewall height7.03 in (178.5 mm)7.50 in (190.5 mm)+0.47 in (+12.0 mm)+6.72%
Section width10.04 in (255.0 mm)9.50 in (241.3 mm)−0.54 in (−13.7 mm)−5.37%
Circumference91.28 in (2,318.5 mm)94.25 in (2,393.9 mm)+2.97 in (+75.4 mm)+3.25%
Revolutions per mile694672−22−3.15%
True speed with the speedometer calibrated for 255/70R15
IndicatedActual on 255/70R15Actual on 30x9.50R15Difference
30 mph30.0 mph31.0 mph+1.0 mph
60 mph60.0 mph62.0 mph+2.0 mph
75 mph75.0 mph77.4 mph+2.4 mph

Ride-height change: 30x9.50R15 raises the vehicle by 0.47 in (12.0 mm) at the axle (half the +24.0 mm diameter difference).

30x9.50R15 is 0.94 in taller than 255/70R15 and 14 mm narrower, a 3.25% difference in overall diameter.

With the speedometer calibrated for 255/70R15, an indicated 60 mph is actually 62.0 mph on 30x9.50R15, and revolutions per mile change from 694 to 672.

Both sizes mount on 15-inch wheels.

Size pages

Other comparisons with 255/70R15

Other comparisons with 30x9.50R15

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