The formula used by this calculator
Total investment equals purchase price plus transport, repairs and preparation, fees and registration, holding cost, and other direct vehicle costs. Estimated vehicle profit equals sale price minus total investment.
ROI compares that estimated profit with total investment. Gross margin compares estimated profit with the sale price. These percentages answer different questions, so the calculator keeps both visible.
Record costs when they happen
A calculator is useful for a quick decision, but accurate history depends on daily capture. Repair invoices, transport charges, fees, parts, and detailing are easiest to miss when the dealership waits until sale day to reconstruct the deal.