TireSizesFinder

265/40R18 vs 265/30R20

265/30R20 is 0.09 in (2.2 mm) shorter than 265/40R18, a −0.33% diameter difference. With the speedometer set for 265/40R18, an indicated 60 mph is 59.8 mph on 265/30R20.

Is this a safe swap?

Diameter difference −0.33%: under 1%

By overall diameter alone, 265/30R20 is a near match for 265/40R18; 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: 265/40R18 (26.35 in) vs 265/30R20 (26.26 in)265/40R1826.35 in × 10.43 in265/30R2026.26 in × 10.43 in
Side profile and tread width, drawn to the same scale.
265/40R18 vs 265/30R20 differences
Measurement265/40R18265/30R20Difference% diff
Overall diameter26.35 in (669.2 mm)26.26 in (667.0 mm)−0.09 in (−2.2 mm)−0.33%
Sidewall height4.17 in (106.0 mm)3.13 in (79.5 mm)−1.04 in (−26.5 mm)−25.00%
Section width10.43 in (265.0 mm)10.43 in (265.0 mm)0.00 in (0.0 mm)0.00%
Circumference82.77 in (2,102.4 mm)82.50 in (2,095.4 mm)−0.27 in (−6.9 mm)−0.33%
Revolutions per mile765768+3+0.33%
True speed with the speedometer calibrated for 265/40R18
IndicatedActual on 265/40R18Actual on 265/30R20Difference
30 mph30.0 mph29.9 mph−0.1 mph
60 mph60.0 mph59.8 mph−0.2 mph
75 mph75.0 mph74.8 mph−0.2 mph

Ride-height change: 265/30R20 lowers the vehicle by 0.04 in (1.1 mm) at the axle (half the −2.2 mm diameter difference).

265/30R20 is 0.09 in shorter than 265/40R18 and the same width, a 0.33% difference in overall diameter.

With the speedometer calibrated for 265/40R18, an indicated 60 mph is actually 59.8 mph on 265/30R20, and revolutions per mile change from 765 to 768.

265/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 265/40R18.

Size pages

Other comparisons with 265/40R18

Other comparisons with 265/30R20

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