245/70R16 vs 255/55R18

255/55R18 is 0.46 in (11.7 mm) shorter than 245/70R16, a −1.56% diameter difference. With the speedometer set for 245/70R16, an indicated 60 mph is 59.1 mph on 255/55R18.

Is this a safe swap?

Diameter difference −1.56%: within the ±3% guideline

By overall diameter alone, 255/55R18 falls inside the ±3% range commonly used as a guideline when replacing 245/70R16. 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.

245/70R16 vs 255/55R18 differences
Measurement	245/70R16	255/55R18	Difference	% diff
Overall diameter	29.50 in (749.4 mm)	29.04 in (737.7 mm)	−0.46 in (−11.7 mm)	−1.56%
Sidewall height	6.75 in (171.5 mm)	5.52 in (140.3 mm)	−1.23 in (−31.2 mm)	−18.22%
Section width	9.65 in (245.0 mm)	10.04 in (255.0 mm)	+0.39 in (+10.0 mm)	+4.08%
Circumference	92.69 in (2,354.3 mm)	91.24 in (2,317.6 mm)	−1.45 in (−36.8 mm)	−1.56%
Revolutions per mile	684	694	+11	+1.59%

True speed with the speedometer calibrated for 245/70R16
Indicated	Actual on 245/70R16	Actual on 255/55R18	Difference
30 mph	30.0 mph	29.5 mph	−0.5 mph
60 mph	60.0 mph	59.1 mph	−0.9 mph
75 mph	75.0 mph	73.8 mph	−1.2 mph

Ride-height change: 255/55R18 lowers the vehicle by 0.23 in (5.8 mm) at the axle (half the −11.7 mm diameter difference).

255/55R18 is 0.46 in shorter than 245/70R16 and 10 mm wider, a 1.56% difference in overall diameter.

With the speedometer calibrated for 245/70R16, an indicated 60 mph is actually 59.1 mph on 255/55R18, and revolutions per mile change from 684 to 694.

255/55R18 mounts on a 18-inch wheel, a plus-2 fitment from the 16-inch wheel of 245/70R16.

Size pages

Other comparisons with 245/70R16

Other comparisons with 255/55R18

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