Car Savings Calculator
Do not include types in variable names

carCostP
       ^


Remove that P (probably stands for a number type?) the compiler takes care of types for you.

Avoid so many variables

You can avoid those four variables altogether, you may just convert to double as soon as you read the vars, like this: 

double carCost = Double.parseDouble(
     JOptionPane.showInputDialog("What is the total cost of the car?"));