The MC-6 was MCI's and Greyhound's first experiment with a 102-inch wide intercity bus. Two prototypes were built in 1967 and production began in 1969 for a total of 100.
Many states balked in the 1970s at providing operating approval for the buses. Fifteen of the MC-6 buses spent their entire Greyhound careers in Canada. The 85 U.S. MC-6s first operated on the East Coast, but were later sent to California to finish their Greyhound careers on the West Coast. The engines in the California buses were changed to 8V71s and the manual transmissions were changed to Allison automatics in 1977. The 15 Canadian buses retained their 12V71 engines and manual transmissions. Greyhound retired them in 1980 and they were sold to independent operators.
Information from the Bus World Encyclopedia of Buses - 1988