The class structure was changed to allow certain cars to be moved out of their displacement class, based on past performance. The nominal class structure was as follows:[1]
^A C and D Modified were classified together at Marlboro; the combined class was won by Fred Gamble's DM Maserati Tipo 61. The highest-finishing CM car was Bud Faust's Ferrari-Chevrolet in 2nd.
^B F Modified were classified with E Modified at Marlboro.
^C F Modified were classified with E Modified at VIR.
^D I Modified were classified with H Modified at Cumberland.
^E C Modified were classified with B Modified at the Road America June Sprints.
^F E Modified were classified with D Modified at Lime Rock Park.
^H E and F Modified were classified together at Bridgehampton (August); the combined class was won by Bob Bucher's FM Porsche. The highest-finishing EM car was Henry Schwartz's Porsche in 3rd.
^I C and D Modified were classified together at Thompson; the combined class was won by George Constantine's DM Ferrari.
