205/40R18 vs 245/35R18

245/35R18 is 0.30 in (7.5 mm) taller than 205/40R18, a +1.21% diameter difference. With the speedometer set for 205/40R18, an indicated 60 mph is 60.7 mph on 245/35R18.

Is this a safe swap?

Diameter difference +1.21%: within the ±3% guideline

By overall diameter alone, 245/35R18 falls inside the ±3% range commonly used as a guideline when replacing 205/40R18. 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.

205/40R18 vs 245/35R18 differences
Measurement	205/40R18	245/35R18	Difference	% diff
Overall diameter	24.46 in (621.2 mm)	24.75 in (628.7 mm)	+0.30 in (+7.5 mm)	+1.21%
Sidewall height	3.23 in (82.0 mm)	3.38 in (85.8 mm)	+0.15 in (+3.8 mm)	+4.57%
Section width	8.07 in (205.0 mm)	9.65 in (245.0 mm)	+1.57 in (+40.0 mm)	+19.51%
Circumference	76.83 in (1,951.6 mm)	77.76 in (1,975.1 mm)	+0.93 in (+23.6 mm)	+1.21%
Revolutions per mile	825	815	−10	−1.19%

True speed with the speedometer calibrated for 205/40R18
Indicated	Actual on 205/40R18	Actual on 245/35R18	Difference
30 mph	30.0 mph	30.4 mph	+0.4 mph
60 mph	60.0 mph	60.7 mph	+0.7 mph
75 mph	75.0 mph	75.9 mph	+0.9 mph

Ride-height change: 245/35R18 raises the vehicle by 0.15 in (3.8 mm) at the axle (half the +7.5 mm diameter difference).

245/35R18 is 0.30 in taller than 205/40R18 and 40 mm wider, a 1.21% difference in overall diameter.

With the speedometer calibrated for 205/40R18, an indicated 60 mph is actually 60.7 mph on 245/35R18, and revolutions per mile change from 825 to 815.

Both sizes mount on 18-inch wheels.

Size pages

Other comparisons with 205/40R18

Other comparisons with 245/35R18

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