The CarSelector widget appears to use a hard-coded 7x7 grid, allowing 49 cars to be chosen from.
Now that iRacing officially supports 60 car fields, this widget occasionally causes cars to be unselectable. Changing the widget to present an 8x8 grid (64 cars) would correct this.
Another option is to allow the dimensions of the widget to be customizable, such as in the properties file. For example, an official series race that only allows up to 30 cars could make do with a 6x6 grid, which would allow the buttons to be larger.