Suregrip- the 5.5`s have a backspace of 3.5 in. and the 6.5`s have a backspace of 3.75. They `re both 14" wheels. The 5.5`s would have D-70/14`s tires mounted on them and the 6.5`s would have E-70/14`s. Most all of the Formula-S Cudas and GTS Darts came with the 6.5 rims. HTH.