TireSizesFinder

33x12.50R18 vs 305/55R20

305/55R20 is 0.21 in (5.3 mm) taller than 33x12.50R18, a +0.63% diameter difference. With the speedometer set for 33x12.50R18, an indicated 60 mph is 60.4 mph on 305/55R20.

Is this a safe swap?

Diameter difference +0.63%: under 1%

By overall diameter alone, 305/55R20 is a near match for 33x12.50R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 33x12.50R18 (33.00 in) vs 305/55R20 (33.21 in)33x12.50R1833.00 in × 12.50 in305/55R2033.21 in × 12.01 in
Side profile and tread width, drawn to the same scale.
33x12.50R18 vs 305/55R20 differences
Measurement33x12.50R18305/55R20Difference% diff
Overall diameter33.00 in (838.2 mm)33.21 in (843.5 mm)+0.21 in (+5.3 mm)+0.63%
Sidewall height7.50 in (190.5 mm)6.60 in (167.8 mm)−0.90 in (−22.7 mm)−11.94%
Section width12.50 in (317.5 mm)12.01 in (305.0 mm)−0.49 in (−12.5 mm)−3.94%
Circumference103.67 in (2,633.3 mm)104.33 in (2,649.9 mm)+0.66 in (+16.7 mm)+0.63%
Revolutions per mile611607−4−0.63%
True speed with the speedometer calibrated for 33x12.50R18
IndicatedActual on 33x12.50R18Actual on 305/55R20Difference
30 mph30.0 mph30.2 mph+0.2 mph
60 mph60.0 mph60.4 mph+0.4 mph
75 mph75.0 mph75.5 mph+0.5 mph

Ride-height change: 305/55R20 raises the vehicle by 0.10 in (2.6 mm) at the axle (half the +5.3 mm diameter difference).

305/55R20 is 0.21 in taller than 33x12.50R18 and 13 mm narrower, a 0.63% difference in overall diameter.

With the speedometer calibrated for 33x12.50R18, an indicated 60 mph is actually 60.4 mph on 305/55R20, and revolutions per mile change from 611 to 607.

305/55R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 33x12.50R18.

Size pages

Other comparisons with 33x12.50R18

Other comparisons with 305/55R20

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