detect boundary in image which is not a perfect circle

i am trying to detect a vehicle tyre

my sample image is

i am using CircularHough_Grd func. it is detecting the inner rim perfectly. as shown below

but it cannot detect the outer portion as it is not perfectly circular, any suggestions.

[Mostafa]

Starting from the second image, I think you can apply some filters to remove the background and then proceed with the edge detecting from there.