Your $0.39/mi figure appears to be derived from dividing your total yearly car costs (both fixed and variable costs) by your total mileage driven, and it is not a figure that represents the rate for each additional mile driven, or conversely the amount that would have been saved for each mile not driven.

That figure wouldn't accurately represent the savings you would get by reducing your car mileage by 2000 miles a year,since your major fixed costs (depreciation, insurance, registration) would not be reduced at all. Outside of fuel costs and parking costs, if any, at work site you would not see much savings at all.