235/45R18 vs 265/30R20

265/30R20 is 0.07 in (1.7 mm) shorter than 235/45R18, a −0.25% diameter difference. With the speedometer set for 235/45R18, an indicated 60 mph is 59.8 mph on 265/30R20.

Is this a safe swap?

Diameter difference −0.25%: under 1%

By overall diameter alone, 265/30R20 is a near match for 235/45R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

Side profile and tread width, drawn to the same scale.

235/45R18 vs 265/30R20 differences
Measurement	235/45R18	265/30R20	Difference	% diff
Overall diameter	26.33 in (668.7 mm)	26.26 in (667.0 mm)	−0.07 in (−1.7 mm)	−0.25%
Sidewall height	4.16 in (105.8 mm)	3.13 in (79.5 mm)	−1.03 in (−26.2 mm)	−24.82%
Section width	9.25 in (235.0 mm)	10.43 in (265.0 mm)	+1.18 in (+30.0 mm)	+12.77%
Circumference	82.71 in (2,100.8 mm)	82.50 in (2,095.4 mm)	−0.21 in (−5.3 mm)	−0.25%
Revolutions per mile	766	768	+2	+0.25%

True speed with the speedometer calibrated for 235/45R18
Indicated	Actual on 235/45R18	Actual on 265/30R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.8 mph	−0.2 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 265/30R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

265/30R20 is 0.07 in shorter than 235/45R18 and 30 mm wider, a 0.25% difference in overall diameter.

With the speedometer calibrated for 235/45R18, an indicated 60 mph is actually 59.8 mph on 265/30R20, and revolutions per mile change from 766 to 768.

265/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 235/45R18.

Size pages

Other comparisons with 235/45R18

Other comparisons with 265/30R20

Source: Calculated from standard tire geometry formulas. Verified 2026-09-27. How these numbers are calculated.