Here is a json with a list of all iRacing default classes.

iRacing Class IDs

[
  {
    "Name": "Audi 90 GTO",
    "ID": 83,
    "CarsInClass": [
      {
        "Name": "Audi 90 GTO",
        "ID": 76
      }
    ]
  },
  {
    "Name": "Cadillac CTS-VR",
    "ID": 45,
    "CarsInClass": [
      {
        "Name": "Cadillac CTS-V Racecar",
        "ID": 41
      }
    ]
  },
  {
    "Name": "Camping World Series",
    "ID": 71,
    "CarsInClass": [
      {
        "Name": "NASCAR Camping World Toyota Tundra",
        "ID": 62
      },
      {
        "Name": "NASCAR Camping World Chevrolet Silverado",
        "ID": 63
      }
    ]
  },
  {
    "Name": "Corvette C7 Daytona Prototype",
    "ID": 77,
    "CarsInClass": [
      {
        "Name": "Chevrolet Corvette C7 Daytona Prototype",
        "ID": 70
      }
    ]
  },
  {
    "Name": "Dallara DW12",
    "ID": 63,
    "CarsInClass": [
      {
        "Name": "Dallara DW12",
        "ID": 57
      }
    ]
  },
  {
    "Name": "Dallara F3",
    "ID": 870,
    "CarsInClass": [
      {
        "Name": "Dallara F3",
        "ID": 106
      }
    ]
  },
  {
    "Name": "Dallara IndyCar",
    "ID": 26,
    "CarsInClass": [
      {
        "Name": "Indycar Dallara - 2011",
        "ID": 29
      }
    ]
  },
  {
    "Name": "Dallara IR18",
    "ID": 117,
    "CarsInClass": [
      {
        "Name": "Dallara IR18",
        "ID": 99
      }
    ]
  },
  {
    "Name": "Dirt Late Model - Limited",
    "ID": 85,
    "CarsInClass": [
      {
        "Name": "Dirt Late Model - Limited",
        "ID": 78
      }
    ]
  },
  {
    "Name": "Dirt Late Model - Pro",
    "ID": 90,
    "CarsInClass": [
      {
        "Name": "Dirt Late Model - Pro",
        "ID": 83
      }
    ]
  },
  {
    "Name": "Dirt Late Model - Super",
    "ID": 91,
    "CarsInClass": [
      {
        "Name": "Dirt Late Model - Super",
        "ID": 84
      }
    ]
  },
  {
    "Name": "Dirt Legends",
    "ID": 89,
    "CarsInClass": [
      {
        "Name": "Dirt Legends Ford '34 Coupe",
        "ID": 82
      }
    ]
  },
  {
    "Name": "Dirt Midget",
    "ID": 104,
    "CarsInClass": [
      {
        "Name": "Dirt Midget",
        "ID": 96
      }
    ]
  },
  {
    "Name": "Dirt SprintCar - 305",
    "ID": 87,
    "CarsInClass": [
      {
        "Name": "Dirt Sprint Car - 305",
        "ID": 80
      }
    ]
  },
  {
    "Name": "Dirt SprintCar - 360",
    "ID": 92,
    "CarsInClass": [
      {
        "Name": "Dirt Sprint Car - 360",
        "ID": 85
      }
    ]
  },
  {
    "Name": "Dirt SprintCar - 360wingless",
    "ID": 96,
    "CarsInClass": [
      {
        "Name": "Dirt Sprint Car - 360 Non-Winged",
        "ID": 87
      }
    ]
  },
  {
    "Name": "Dirt Sprintcar - 410",
    "ID": 93,
    "CarsInClass": [
      {
        "Name": "Dirt Sprint Car - 410",
        "ID": 86
      }
    ]
  },
  {
    "Name": "Dirt SprintCar - 410wingless",
    "ID": 94,
    "CarsInClass": [
      {
        "Name": "Dirt Sprint Car - 410 Non-Winged",
        "ID": 89
      }
    ]
  },
  {
    "Name": "Dirt Street Stock",
    "ID": 86,
    "CarsInClass": [
      {
        "Name": "Dirt Street Stock",
        "ID": 79
      }
    ]
  },
  {
    "Name": "Dirt UMP Modified",
    "ID": 103,
    "CarsInClass": [
      {
        "Name": "Dirt UMP Modified",
        "ID": 95
      }
    ]
  },
  {
    "Name": "Ferrari 488 GT3",
    "ID": 102,
    "CarsInClass": [
      {
        "Name": "Ferrari 488 GT3",
        "ID": 94
      }
    ]
  },
  {
    "Name": "Ford GT",
    "ID": 41,
    "CarsInClass": [
      {
        "Name": "Ford GT",
        "ID": 40
      }
    ]
  },
  {
    "Name": "Ford Mustang FR500S",
    "ID": 28,
    "CarsInClass": [
      {
        "Name": "Ford Mustang FR500S",
        "ID": 30
      }
    ]
  },
  {
    "Name": "Formula Renault 2.0",
    "ID": 81,
    "CarsInClass": [
      {
        "Name": "Formula Renault 2.0",
        "ID": 74
      }
    ]
  },
  {
    "Name": "Formula Renault 3.5",
    "ID": 871,
    "CarsInClass": [
      {
        "Name": "Formula Renault 3.5",
        "ID": 105
      }
    ]
  },
  {
    "Name": "GT1 Class",
    "ID": 73,
    "CarsInClass": [
      {
        "Name": "Chevrolet Corvette C6.R GT1",
        "ID": 26
      },
      {
        "Name": "Aston Martin DBR9 GT1",
        "ID": 64
      }
    ]
  },
  {
    "Name": "GT3 Class",
    "ID": 59,
    "CarsInClass": [
      {
        "Name": "McLaren MP4-12C GT3",
        "ID": 43
      },
      {
        "Name": "BMW Z4 GT3",
        "ID": 55
      },
      {
        "Name": "Ford GT GT3",
        "ID": 59
      },
      {
        "Name": "Mercedes AMG GT3",
        "ID": 72
      },
      {
        "Name": "Audi R8 LMS",
        "ID": 73
      },
      {
        "Name": "Ferrari 488 GT3",
        "ID": 94
      }
    ]
  },
  {
    "Name": "GTE Class",
    "ID": 100,
    "CarsInClass": [
      {
        "Name": "Ford GT - 2017",
        "ID": 92
      },
      {
        "Name": "Ferrari 488 GTE",
        "ID": 93
      },
      {
        "Name": "Porsche 911 RSR",
        "ID": 102
      }
    ]
  },
  {
    "Name": "HPD ARX-01c",
    "ID": 40,
    "CarsInClass": [
      {
        "Name": "HPD ARX-01c",
        "ID": 39
      }
    ]
  },
  {
    "Name": "IMSA2 GT3",
    "ID": 473,
    "CarsInClass": [
      {
        "Name": "Mercedes AMG GT3",
        "ID": 72
      },
      {
        "Name": "Audi R8 LMS",
        "ID": 73
      }
    ]
  },
  {
    "Name": "K&N Series",
    "ID": 21,
    "CarsInClass": [
      {
        "Name": "NASCAR K&N Pro Chevrolet Impala",
        "ID": 24
      }
    ]
  },
  {
    "Name": "Kia Optima",
    "ID": 48,
    "CarsInClass": [
      {
        "Name": "Kia Optima",
        "ID": 44
      }
    ]
  },
  {
    "Name": "Legends",
    "ID": 5,
    "CarsInClass": [
      {
        "Name": "Legends Ford '34 Coupe",
        "ID": 5
      }
    ]
  },
  {
    "Name": "LMP1 Class",
    "ID": 116,
    "CarsInClass": [
      {
        "Name": "Audi R18",
        "ID": 98
      },
      {
        "Name": "Porsche 919",
        "ID": 100
      }
    ]
  },
  {
    "Name": "Lotus 49",
    "ID": 46,
    "CarsInClass": [
      {
        "Name": "Lotus 49",
        "ID": 42
      }
    ]
  },
  {
    "Name": "Lotus 79",
    "ID": 22,
    "CarsInClass": [
      {
        "Name": "Lotus 79",
        "ID": 25
      }
    ]
  },
  {
    "Name": "Mazda MX-5 Cup",
    "ID": 32,
    "CarsInClass": [
      {
        "Name": "Mazda MX-5 Cup - 2015",
        "ID": 34
      }
    ]
  },
  {
    "Name": "Mazda MX-5 Cup 2016",
    "ID": 74,
    "CarsInClass": [
      {
        "Name": "Global Mazda MX-5 Cup",
        "ID": 67
      }
    ]
  },
  {
    "Name": "McLaren MP4-30",
    "ID": 78,
    "CarsInClass": [
      {
        "Name": "McLaren MP4-30",
        "ID": 71
      }
    ]
  },
  {
    "Name": "Monte Carlo SS",
    "ID": 14,
    "CarsInClass": [
      {
        "Name": "Chevrolet Monte Carlo SS",
        "ID": 12
      }
    ]
  },
  {
    "Name": "Nissan GTP",
    "ID": 84,
    "CarsInClass": [
      {
        "Name": "Nissan GTP ZX-T",
        "ID": 77
      }
    ]
  },
  {
    "Name": "Porsche 911 GT3 Cup",
    "ID": 95,
    "CarsInClass": [
      {
        "Name": "Porsche 911 GT3 Cup (991)",
        "ID": 88
      }
    ]
  },
  {
    "Name": "Pro Mazda",
    "ID": 4,
    "CarsInClass": [
      {
        "Name": "Pro Mazda",
        "ID": 4
      }
    ]
  },
  {
    "Name": "Radical SR8",
    "ID": 15,
    "CarsInClass": [
      {
        "Name": "Radical SR8",
        "ID": 13
      }
    ]
  },
  {
    "Name": "Rallycross",
    "ID": 105,
    "CarsInClass": [
      {
        "Name": "Ford Fiesta RS WRC",
        "ID": 81
      },
      {
        "Name": "VW Beetle",
        "ID": 91
      },
      {
        "Name": "Subaru WRX STI",
        "ID": 101
      }
    ]
  },
  {
    "Name": "Ruf RT 12R Track",
    "ID": 55,
    "CarsInClass": [
      {
        "Name": "Ruf RT 12R Track",
        "ID": 50
      }
    ]
  },
  {
    "Name": "Silver Crown",
    "ID": 16,
    "CarsInClass": [
      {
        "Name": "Silver Crown",
        "ID": 18
      }
    ]
  },
  {
    "Name": "Silverado",
    "ID": 17,
    "CarsInClass": [
      {
        "Name": "NASCAR Truck Series Chevrolet Silverado - 2013",
        "ID": 20
      }
    ]
  },
  {
    "Name": "SK Modified",
    "ID": 2,
    "CarsInClass": [
      {
        "Name": "Modified - SK",
        "ID": 2
      }
    ]
  },
  {
    "Name": "Skip Barber Race Series",
    "ID": 1,
    "CarsInClass": [
      {
        "Name": "Skip Barber Formula 2000",
        "ID": 1
      }
    ]
  },
  {
    "Name": "Solstice",
    "ID": 3,
    "CarsInClass": [
      {
        "Name": "Pontiac Solstice",
        "ID": 3
      }
    ]
  },
  {
    "Name": "Spec Racer Ford",
    "ID": 20,
    "CarsInClass": [
      {
        "Name": "SCCA Spec Racer Ford",
        "ID": 23
      }
    ]
  },
  {
    "Name": "Sprint Car",
    "ID": 35,
    "CarsInClass": [
      {
        "Name": "Sprint Car",
        "ID": 37
      }
    ]
  },
  {
    "Name": "Sprint Cup",
    "ID": 50,
    "CarsInClass": [
      {
        "Name": "NASCAR Monster Energy Cup Ford Fusion",
        "ID": 46
      },
      {
        "Name": "NASCAR Monster Energy Cup Toyota Camry",
        "ID": 56
      },
      {
        "Name": "NASCAR Monster Energy Cup Chevrolet Camaro ZL1",
        "ID": 103
      }
    ]
  },
  {
    "Name": "Street Stock",
    "ID": 34,
    "CarsInClass": [
      {
        "Name": "Street Stock",
        "ID": 36
      }
    ]
  },
  {
    "Name": "Super Late Model",
    "ID": 60,
    "CarsInClass": [
      {
        "Name": "Super Late Model",
        "ID": 54
      }
    ]
  },
  {
    "Name": "V8Supercars",
    "ID": 67,
    "CarsInClass": [
      {
        "Name": "Holden Commodore VF V8",
        "ID": 60
      },
      {
        "Name": "Ford Falcon FG V8",
        "ID": 61
      }
    ]
  },
  {
    "Name": "VW Jetta TDI Cup",
    "ID": 24,
    "CarsInClass": [
      {
        "Name": "VW Jetta TDI Cup",
        "ID": 27
      }
    ]
  },
  {
    "Name": "Whelen Modified",
    "ID": 29,
    "CarsInClass": [
      {
        "Name": "Modified - NASCAR Whelen Tour",
        "ID": 31
      }
    ]
  },
  {
    "Name": "Xfinity Series",
    "ID": 57,
    "CarsInClass": [
      {
        "Name": "NASCAR XFINITY Ford Mustang",
        "ID": 51
      },
      {
        "Name": "NASCAR XFINITY Chevrolet Camaro",
        "ID": 58
      },
      {
        "Name": "NASCAR XFINITY Toyota Camry",
        "ID": 69
      }
    ]
  }
]

Source: http://members.iracing.com/jforum/posts/list/3633011.page#11389540