The ratio of blue cars to red cars is given as 11:6. This means that for every 11 blue cars, there are 6 red cars.
Let the number of red cars be \( r \). According to the ratio, we can set up the following relation based on the number of blue cars (which is given as 317):
\[ \frac{11}{6} = \frac{317}{r} \]
To solve for \( r \), we can cross-multiply:
\[ 11r = 6 \times 317 \]
Calculating the right side:
\[ 6 \times 317 = 1902 \]
So we have:
\[ 11r = 1902 \]
Now, divide both sides by 11 to find \( r \):
\[ r = \frac{1902}{11} \approx 173.0909 \]
Rounding this to the nearest whole number, we get:
\[ r \approx 173 \]
Thus, the number of red cars in the parking lot is \( \boxed{173} \).