245/40R17 vs 255/40R17

255/40R17 is 0.31 in (8.0 mm) taller than 245/40R17, a +1.27% diameter difference. With the speedometer set for 245/40R17, an indicated 60 mph is 60.8 mph on 255/40R17.

Is this a safe swap?

Diameter difference +1.27%: within the ±3% guideline

By overall diameter alone, 255/40R17 falls inside the ±3% range commonly used as a guideline when replacing 245/40R17. 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.

245/40R17 vs 255/40R17 differences
Measurement	245/40R17	255/40R17	Difference	% diff
Overall diameter	24.72 in (627.8 mm)	25.03 in (635.8 mm)	+0.31 in (+8.0 mm)	+1.27%
Sidewall height	3.86 in (98.0 mm)	4.02 in (102.0 mm)	+0.16 in (+4.0 mm)	+4.08%
Section width	9.65 in (245.0 mm)	10.04 in (255.0 mm)	+0.39 in (+10.0 mm)	+4.08%
Circumference	77.65 in (1,972.3 mm)	78.64 in (1,997.4 mm)	+0.99 in (+25.1 mm)	+1.27%
Revolutions per mile	816	806	−10	−1.26%

True speed with the speedometer calibrated for 245/40R17
Indicated	Actual on 245/40R17	Actual on 255/40R17	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.8 mph	+0.8 mph
75 mph	75.0 mph	76.0 mph	+1.0 mph

Ride-height change: 255/40R17 raises the vehicle by 0.16 in (4.0 mm) at the axle (half the +8.0 mm diameter difference).

255/40R17 is 0.31 in taller than 245/40R17 and 10 mm wider, a 1.27% difference in overall diameter.

With the speedometer calibrated for 245/40R17, an indicated 60 mph is actually 60.8 mph on 255/40R17, and revolutions per mile change from 816 to 806.

Both sizes mount on 17-inch wheels.

Size pages

Other comparisons with 245/40R17

Other comparisons with 255/40R17

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