TireSizesFinder

255/40R17 vs 215/40R18

215/40R18 is 0.26 in (6.6 mm) shorter than 255/40R17, a −1.04% diameter difference. With the speedometer set for 255/40R17, an indicated 60 mph is 59.4 mph on 215/40R18.

Is this a safe swap?

Diameter difference −1.04%: within the ±3% guideline

By overall diameter alone, 215/40R18 falls inside the ±3% range commonly used as a guideline when replacing 255/40R17. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 255/40R17 (25.03 in) vs 215/40R18 (24.77 in)255/40R1725.03 in × 10.04 in215/40R1824.77 in × 8.46 in
Side profile and tread width, drawn to the same scale.
255/40R17 vs 215/40R18 differences
Measurement255/40R17215/40R18Difference% diff
Overall diameter25.03 in (635.8 mm)24.77 in (629.2 mm)−0.26 in (−6.6 mm)−1.04%
Sidewall height4.02 in (102.0 mm)3.39 in (86.0 mm)−0.63 in (−16.0 mm)−15.69%
Section width10.04 in (255.0 mm)8.46 in (215.0 mm)−1.57 in (−40.0 mm)−15.69%
Circumference78.64 in (1,997.4 mm)77.82 in (1,976.7 mm)−0.82 in (−20.7 mm)−1.04%
Revolutions per mile806814+8+1.05%
True speed with the speedometer calibrated for 255/40R17
IndicatedActual on 255/40R17Actual on 215/40R18Difference
30 mph30.0 mph29.7 mph−0.3 mph
60 mph60.0 mph59.4 mph−0.6 mph
75 mph75.0 mph74.2 mph−0.8 mph

Ride-height change: 215/40R18 lowers the vehicle by 0.13 in (3.3 mm) at the axle (half the −6.6 mm diameter difference).

215/40R18 is 0.26 in shorter than 255/40R17 and 40 mm narrower, a 1.04% difference in overall diameter.

With the speedometer calibrated for 255/40R17, an indicated 60 mph is actually 59.4 mph on 215/40R18, and revolutions per mile change from 806 to 814.

215/40R18 mounts on a 18-inch wheel, a plus-1 fitment from the 17-inch wheel of 255/40R17.

Size pages

Other comparisons with 255/40R17

Other comparisons with 215/40R18

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