DIRTI, is a mostly fixed significant cost of ownership. D=depreciation, what will the depreciation of the two vehicles be. I=interest, what is the interest and opportunity cost of having the two vehicles (include DIRTI in the calculations) R=repairs,(tires, oil changes,car washing and other maintenance costs which are variable depending on miles driven, T=taxes (licenses and sales taxes on the purchase price) I=insurance.
The savings in variable costs,(gas)must offset DIRTI to justify have a second vehicle, plus the desire to have that second vehicle.