255/55R18 vs 295/40R20

295/40R20 is 0.25 in (6.3 mm) taller than 255/55R18, a +0.85% diameter difference. With the speedometer set for 255/55R18, an indicated 60 mph is 60.5 mph on 295/40R20.

Is this a safe swap?

Diameter difference +0.85%: under 1%

By overall diameter alone, 295/40R20 is a near match for 255/55R18; 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.

255/55R18 vs 295/40R20 differences
Measurement	255/55R18	295/40R20	Difference	% diff
Overall diameter	29.04 in (737.7 mm)	29.29 in (744.0 mm)	+0.25 in (+6.3 mm)	+0.85%
Sidewall height	5.52 in (140.3 mm)	4.65 in (118.0 mm)	−0.88 in (−22.2 mm)	−15.86%
Section width	10.04 in (255.0 mm)	11.61 in (295.0 mm)	+1.57 in (+40.0 mm)	+15.69%
Circumference	91.24 in (2,317.6 mm)	92.02 in (2,337.3 mm)	+0.78 in (+19.8 mm)	+0.85%
Revolutions per mile	694	689	−6	−0.85%

True speed with the speedometer calibrated for 255/55R18
Indicated	Actual on 255/55R18	Actual on 295/40R20	Difference
30 mph	30.0 mph	30.3 mph	+0.3 mph
60 mph	60.0 mph	60.5 mph	+0.5 mph
75 mph	75.0 mph	75.6 mph	+0.6 mph

Ride-height change: 295/40R20 raises the vehicle by 0.12 in (3.1 mm) at the axle (half the +6.3 mm diameter difference).

295/40R20 is 0.25 in taller than 255/55R18 and 40 mm wider, a 0.85% difference in overall diameter.

With the speedometer calibrated for 255/55R18, an indicated 60 mph is actually 60.5 mph on 295/40R20, and revolutions per mile change from 694 to 689.

295/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/55R18.

Size pages

Other comparisons with 255/55R18

Other comparisons with 295/40R20

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