255/45R18 vs 295/30R20

295/30R20 is 0.07 in (1.7 mm) shorter than 255/45R18, a −0.25% diameter difference. With the speedometer set for 255/45R18, an indicated 60 mph is 59.9 mph on 295/30R20.

Is this a safe swap?

Diameter difference −0.25%: under 1%

By overall diameter alone, 295/30R20 is a near match for 255/45R18; 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.

255/45R18 vs 295/30R20 differences
Measurement	255/45R18	295/30R20	Difference	% diff
Overall diameter	27.04 in (686.7 mm)	26.97 in (685.0 mm)	−0.07 in (−1.7 mm)	−0.25%
Sidewall height	4.52 in (114.8 mm)	3.48 in (88.5 mm)	−1.03 in (−26.2 mm)	−22.88%
Section width	10.04 in (255.0 mm)	11.61 in (295.0 mm)	+1.57 in (+40.0 mm)	+15.69%
Circumference	84.93 in (2,157.3 mm)	84.72 in (2,152.0 mm)	−0.21 in (−5.3 mm)	−0.25%
Revolutions per mile	746	748	+2	+0.25%

True speed with the speedometer calibrated for 255/45R18
Indicated	Actual on 255/45R18	Actual on 295/30R20	Difference
30 mph	30.0 mph	29.9 mph	−0.1 mph
60 mph	60.0 mph	59.9 mph	−0.1 mph
75 mph	75.0 mph	74.8 mph	−0.2 mph

Ride-height change: 295/30R20 lowers the vehicle by 0.03 in (0.9 mm) at the axle (half the −1.7 mm diameter difference).

295/30R20 is 0.07 in shorter than 255/45R18 and 40 mm wider, a 0.25% difference in overall diameter.

With the speedometer calibrated for 255/45R18, an indicated 60 mph is actually 59.9 mph on 295/30R20, and revolutions per mile change from 746 to 748.

295/30R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 255/45R18.

Size pages

Other comparisons with 255/45R18

Other comparisons with 295/30R20

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