TireSizesFinder

305/30R19 vs 255/30R20

255/30R20 is 0.18 in (4.6 mm) shorter than 305/30R19, a −0.69% diameter difference. With the speedometer set for 305/30R19, an indicated 60 mph is 59.6 mph on 255/30R20.

Is this a safe swap?

Diameter difference −0.69%: under 1%

By overall diameter alone, 255/30R20 is a near match for 305/30R19; 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: 305/30R19 (26.20 in) vs 255/30R20 (26.02 in)305/30R1926.20 in × 12.01 in255/30R2026.02 in × 10.04 in
Side profile and tread width, drawn to the same scale.
305/30R19 vs 255/30R20 differences
Measurement305/30R19255/30R20Difference% diff
Overall diameter26.20 in (665.6 mm)26.02 in (661.0 mm)−0.18 in (−4.6 mm)−0.69%
Sidewall height3.60 in (91.5 mm)3.01 in (76.5 mm)−0.59 in (−15.0 mm)−16.39%
Section width12.01 in (305.0 mm)10.04 in (255.0 mm)−1.97 in (−50.0 mm)−16.39%
Circumference82.32 in (2,091.0 mm)81.76 in (2,076.6 mm)−0.57 in (−14.5 mm)−0.69%
Revolutions per mile770775+5+0.70%
True speed with the speedometer calibrated for 305/30R19
IndicatedActual on 305/30R19Actual on 255/30R20Difference
30 mph30.0 mph29.8 mph−0.2 mph
60 mph60.0 mph59.6 mph−0.4 mph
75 mph75.0 mph74.5 mph−0.5 mph

Ride-height change: 255/30R20 lowers the vehicle by 0.09 in (2.3 mm) at the axle (half the −4.6 mm diameter difference).

255/30R20 is 0.18 in shorter than 305/30R19 and 50 mm narrower, a 0.69% difference in overall diameter.

With the speedometer calibrated for 305/30R19, an indicated 60 mph is actually 59.6 mph on 255/30R20, and revolutions per mile change from 770 to 775.

255/30R20 mounts on a 20-inch wheel, a plus-1 fitment from the 19-inch wheel of 305/30R19.

Size pages

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