As written, Option 4 you only have a car for 3 years. I'm pretty sure that's not what you meant, but fixing it makes the math below also wrong: if you keep a car that is 7 years old for 10 years, it's an average of 12 years old in that time period.

edit: my mistake, I guess you want to have 3.333 cars in 10 years for Option 4. Carry on...