To determine the constant of proportionality (miles per gallon), you can use the formula:
\[ \text{Constant of proportionality} = \frac{\text{Distance traveled}}{\text{Gasoline used}} \]
Given that the car travels 640 miles and uses 32 gallons of gas, you can calculate it as follows:
\[ \text{Constant of proportionality} = \frac{640 \text{ miles}}{32 \text{ gallons}} = 20 \text{ miles per gallon} \]
Therefore, the constant of proportionality is 20 miles per gallon.