The gas tank of Dave's car has a capacity of 12 gallons. The tank was \( \frac{3}{8} \) full before Dave filled it to capacity. It cost him \( \$ 2.50 \) per gallon of gas. How much did Dave spend, in dollars, to fill the tank to capacity?

Before filling the tank, the amount of gas Dave had can be calculated as follows: \( 12 \text{ gallons} \times \frac{3}{8} = 4.5 \text{ gallons} \). To fill the tank to its full capacity, he needed to add \( 12 \text{ gallons} - 4.5 \text{ gallons} = 7.5 \text{ gallons} \) of gas. At a cost of \( \$ 2.50 \) per gallon, Dave’s total expenditure for the gas is calculated as follows: \( 7.5 \text{ gallons} \times \$ 2.50 = \$ 18.75 \). So, Dave spent \( \$ 18.75 \) to fill his gas tank to capacity.