The MSHD completed a major rerouting of M-66 around 1944–45. The M-14 designation of the time was decommissioned and replaced with M-66. Starting at Six Lakes, M-66 turned west along M-46 instead of east and then turned south through Stanton and Ionia ending north of Battle Creek. The highway between Lowell and Lakeview was redesignated as M-91. A minor realignment in late 1950 removed two 90° curves near the Osceola–Missaukee county line and replaced them with a pair of sweeping curves.