TireSizesFinder

255/70R18 vs 305/50R20

305/50R20 is 0.05 in (1.2 mm) shorter than 255/70R18, a −0.15% diameter difference. With the speedometer set for 255/70R18, an indicated 60 mph is 59.9 mph on 305/50R20.

Is this a safe swap?

Diameter difference −0.15%: under 1%

By overall diameter alone, 305/50R20 is a near match for 255/70R18; 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: 255/70R18 (32.06 in) vs 305/50R20 (32.01 in)255/70R1832.06 in × 10.04 in305/50R2032.01 in × 12.01 in
Side profile and tread width, drawn to the same scale.
255/70R18 vs 305/50R20 differences
Measurement255/70R18305/50R20Difference% diff
Overall diameter32.06 in (814.2 mm)32.01 in (813.0 mm)−0.05 in (−1.2 mm)−0.15%
Sidewall height7.03 in (178.5 mm)6.00 in (152.5 mm)−1.02 in (−26.0 mm)−14.57%
Section width10.04 in (255.0 mm)12.01 in (305.0 mm)+1.97 in (+50.0 mm)+19.61%
Circumference100.70 in (2,557.9 mm)100.56 in (2,554.1 mm)−0.15 in (−3.8 mm)−0.15%
Revolutions per mile629630+1+0.15%
True speed with the speedometer calibrated for 255/70R18
IndicatedActual on 255/70R18Actual on 305/50R20Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph59.9 mph−0.1 mph
75 mph75.0 mph74.9 mph−0.1 mph

Ride-height change: 305/50R20 lowers the vehicle by 0.02 in (0.6 mm) at the axle (half the −1.2 mm diameter difference).

305/50R20 is 0.05 in shorter than 255/70R18 and 50 mm wider, a 0.15% difference in overall diameter.

With the speedometer calibrated for 255/70R18, an indicated 60 mph is actually 59.9 mph on 305/50R20, and revolutions per mile change from 629 to 630.

305/50R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/70R18.

Size pages

Other comparisons with 255/70R18

Other comparisons with 305/50R20

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