length ratio of objects in an image

I am trying to evaluate the ratio between the diameter of the wheel circles and the wheel-to-wheel distance in the following image. What i know is that the circular borders of the two wheels have the same diameter, and they lie on a common plane, at a same height above the street level. The plane containing the two circular borders is parallel to the symmetry plane of the car chassy (hence is vertical). How can this ratio be evaluated? I am triyng to do this by measuring the angle formed between the lines bitangent to the rims, given that the tangent of this angle is always equal to the ratio between the diameter of the rims and the distance between them and the vanishing point, but how can i be able to evaluate the real world ratio between these two measure?

[Retteprallele95]

eat more parmigiana to solve this

[Image Analyst]

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