I raised a support ticket on this but was directed to this forum so here it goes:
The standard tire size for this car is 180/80R14 front/rear.
This is narrower than the 1965 Mustang GT Coupe, which is 195/70R14, and is in fact the narrowest default size of any of the 1960s-1970s Mustangs present in the game. Looking at documentation for the real car with the 390 motor we see the narrowest tire at 195/75R14.
The next tire size available in game as a custom upgrade is 245/60R14 front and 255/55 R14 rear.
The jump from 180/80R14 to 245/60R14 / 255/55R14 is too high and it leads to the tires being either too skinny or foo wide for a realistic build, creating a bad look for an otherwise awesome car.
Please consider either making the default wider (195, 205), or give us at least one intermediate option in custom tire width between 180 and 245/255.
On a side-note I’d really like to see a Mustang 2 in the game. Like the 78 King Cobra that we used to have.
For anyone looking to have a historic collection of Mustangs by generation, this absence is disappointing. Likewise for the Camaro.