В вашем случае как-то так
$cars = Cars::all();
$brands = $cars->groupBy('brand');
$data = new \Illuminate\Pagination\LengthAwarePaginator(
$brands, $cars->count(), 5, $currentPage
);
foreach($data as $brand => $models) {
echo $brand;
foreach($models as $car) {
echo $car->model;
}
}