255/70R18 vs 33x12.50R18

33x12.50R18 is 0.94 in (24.0 mm) taller than 255/70R18, a +2.95% diameter difference. With the speedometer set for 255/70R18, an indicated 60 mph is 61.8 mph on 33x12.50R18.

Is this a safe swap?

Diameter difference +2.95%: within the ±3% guideline

By overall diameter alone, 33x12.50R18 falls inside the ±3% range commonly used as a guideline when replacing 255/70R18. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

255/70R18 vs 33x12.50R18 differences
Measurement	255/70R18	33x12.50R18	Difference	% diff
Overall diameter	32.06 in (814.2 mm)	33.00 in (838.2 mm)	+0.94 in (+24.0 mm)	+2.95%
Sidewall height	7.03 in (178.5 mm)	7.50 in (190.5 mm)	+0.47 in (+12.0 mm)	+6.72%
Section width	10.04 in (255.0 mm)	12.50 in (317.5 mm)	+2.46 in (+62.5 mm)	+24.51%
Circumference	100.70 in (2,557.9 mm)	103.67 in (2,633.3 mm)	+2.97 in (+75.4 mm)	+2.95%
Revolutions per mile	629	611	−18	−2.86%

True speed with the speedometer calibrated for 255/70R18
Indicated	Actual on 255/70R18	Actual on 33x12.50R18	Difference
30 mph	30.0 mph	30.9 mph	+0.9 mph
60 mph	60.0 mph	61.8 mph	+1.8 mph
75 mph	75.0 mph	77.2 mph	+2.2 mph

Ride-height change: 33x12.50R18 raises the vehicle by 0.47 in (12.0 mm) at the axle (half the +24.0 mm diameter difference).

33x12.50R18 is 0.94 in taller than 255/70R18 and 63 mm wider, a 2.95% difference in overall diameter.

With the speedometer calibrated for 255/70R18, an indicated 60 mph is actually 61.8 mph on 33x12.50R18, and revolutions per mile change from 629 to 611.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 255/70R18

Other comparisons with 33x12.50R18

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.