To find the percent of decrease in the number of cars, we can use the following formula:
\[ \text{Percent Decrease} = \left( \frac{\text{Old Number} - \text{New Number}}{\text{Old Number}} \right) \times 100 \]
In this case, the old number of cars is 64 and the new number is 48.
First, we calculate the difference in the number of cars:
\[ \text{Difference} = 64 - 48 = 16 \]
Next, we plug the values into the percent decrease formula:
\[ \text{Percent Decrease} = \left( \frac{16}{64} \right) \times 100 \]
Calculating \( \frac{16}{64} \):
\[ \frac{16}{64} = 0.25 \]
Now, multiply by 100:
\[ 0.25 \times 100 = 25 \]
Thus, the percent of decrease in the number of cars is \( \boxed{25%} \).