M-66 was extended southerly from Assyria through Battle Creek to the Indiana state line replacing sections of M-78 in 1965. M-66 turned south and west along M-60 and new highway to Colon. The segment of former M-78 not used by M-66 was transferred to local control. The final section of M-78's roadway given to M-66 extended it all the way to the state line, resulting in a north–south trans-peninsular highway from Lake Michigan near Charlevoix to Indiana. M-66 is the only such highway to run the length of the Lower Peninsula. The extension allowed the Green Arrow Association to promote the whole length of the Green Arrow Route with a single highway number. The next year, M-66 was rerouted through Battle Creek to use the completed I-194 freeway.