TireSizesFinder

305/70R18 vs 275/65R20

275/65R20 is 0.74 in (18.7 mm) shorter than 305/70R18, a −2.11% diameter difference. With the speedometer set for 305/70R18, an indicated 60 mph is 58.7 mph on 275/65R20.

Is this a safe swap?

Diameter difference −2.11%: within the ±3% guideline

By overall diameter alone, 275/65R20 falls inside the ±3% range commonly used as a guideline when replacing 305/70R18. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 305/70R18 (34.81 in) vs 275/65R20 (34.07 in)305/70R1834.81 in × 12.01 in275/65R2034.07 in × 10.83 in
Side profile and tread width, drawn to the same scale.
305/70R18 vs 275/65R20 differences
Measurement305/70R18275/65R20Difference% diff
Overall diameter34.81 in (884.2 mm)34.07 in (865.5 mm)−0.74 in (−18.7 mm)−2.11%
Sidewall height8.41 in (213.5 mm)7.04 in (178.8 mm)−1.37 in (−34.7 mm)−16.28%
Section width12.01 in (305.0 mm)10.83 in (275.0 mm)−1.18 in (−30.0 mm)−9.84%
Circumference109.36 in (2,777.8 mm)107.05 in (2,719.0 mm)−2.31 in (−58.7 mm)−2.11%
Revolutions per mile579592+13+2.16%
True speed with the speedometer calibrated for 305/70R18
IndicatedActual on 305/70R18Actual on 275/65R20Difference
30 mph30.0 mph29.4 mph−0.6 mph
60 mph60.0 mph58.7 mph−1.3 mph
75 mph75.0 mph73.4 mph−1.6 mph

Ride-height change: 275/65R20 lowers the vehicle by 0.37 in (9.4 mm) at the axle (half the −18.7 mm diameter difference).

275/65R20 is 0.74 in shorter than 305/70R18 and 30 mm narrower, a 2.11% difference in overall diameter.

With the speedometer calibrated for 305/70R18, an indicated 60 mph is actually 58.7 mph on 275/65R20, and revolutions per mile change from 579 to 592.

275/65R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 305/70R18.

Size pages

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