It probably wasn’t that difficult to enforce. New tires were good for only 12,000 miles, and that was if you took care of them, and each registered car was allowed four for the duration of the war. There were no newly manufactured automobiles for the duration except those ordered by the military, and those were all of the same models that had been introduced in 1941. Of course, gas was severely rationed as well, despite the fact that the US was the world’s largest producer of petroleum, but restricting gasoline made tires last longer, and rubber was the chokepoint.