TireSizesFinder

245/60R18 vs 305/40R20

305/40R20 is 0.03 in (0.8 mm) taller than 245/60R18, a +0.11% diameter difference. With the speedometer set for 245/60R18, an indicated 60 mph is 60.1 mph on 305/40R20.

Is this a safe swap?

Diameter difference +0.11%: under 1%

By overall diameter alone, 305/40R20 is a near match for 245/60R18; the speedometer changes by less than 1%. This check uses diameter only. It does not assess load index, speed rating, wheel width or clearance.

To-scale drawing: 245/60R18 (29.57 in) vs 305/40R20 (29.61 in)245/60R1829.57 in × 9.65 in305/40R2029.61 in × 12.01 in
Side profile and tread width, drawn to the same scale.
245/60R18 vs 305/40R20 differences
Measurement245/60R18305/40R20Difference% diff
Overall diameter29.57 in (751.2 mm)29.61 in (752.0 mm)+0.03 in (+0.8 mm)+0.11%
Sidewall height5.79 in (147.0 mm)4.80 in (122.0 mm)−0.98 in (−25.0 mm)−17.01%
Section width9.65 in (245.0 mm)12.01 in (305.0 mm)+2.36 in (+60.0 mm)+24.49%
Circumference92.91 in (2,360.0 mm)93.01 in (2,362.5 mm)+0.10 in (+2.5 mm)+0.11%
Revolutions per mile682681−1−0.11%
True speed with the speedometer calibrated for 245/60R18
IndicatedActual on 245/60R18Actual on 305/40R20Difference
30 mph30.0 mph30.0 mph0.0 mph
60 mph60.0 mph60.1 mph+0.1 mph
75 mph75.0 mph75.1 mph+0.1 mph

Ride-height change: 305/40R20 raises the vehicle by 0.02 in (0.4 mm) at the axle (half the +0.8 mm diameter difference).

305/40R20 is 0.03 in taller than 245/60R18 and 60 mm wider, a 0.11% difference in overall diameter.

With the speedometer calibrated for 245/60R18, an indicated 60 mph is actually 60.1 mph on 305/40R20, and revolutions per mile change from 682 to 681.

305/40R20 mounts on a 20-inch wheel, a plus-2 fitment from the 18-inch wheel of 245/60R18.

Size pages

Other comparisons with 245/60R18

Other comparisons with 305/40R20

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