The problem trying to fit a car tire inside a motorcycle fender is the tire profile. Car tires have a flat tread area whereas the cycle tire has a curved tread area. The cycle fender matches the curve of the cycle tire.
Now if it's a custom bike with a flat fender the car tire will fit no problemo with the correct wheel.
As the resident Darksider here I can tell you that there are a lot of factors that go into fitment of a CT.
A through understanding of exactly what the numbers in the size mean is the first step.
Rim width is a huge factor.
Sidewall to belt clearance is another big issue. Some bikes have ample room while others are tight to start with.
Feel free to PM me if you wish. I'm no expert but I do have extensive experience with this subject.