Best result for BMW: GPU rendering at 256x256 - 2.04 min
My system:
CPU: Intel 4Core™ i5-6500 @ 3.20GHz
GPU: Nvidia GTX 1050 Ti
OS: Windows 10 64bit, Blender 2.79a
tiles: 512
Time: 2.06 (GPU - CUDA)
Time: > 8.00 (CPU)
tiles: 256
Time: 2.04 (GPU - CUDA)
Time: 6.00 (CPU)
tiles: 128
Time: 2.41 (GPU - CUDA)
Time: 6.10 (CPU)
tiles: 64
Time: 3.28 (GPU - CUDA)
Time: 6.00 (CPU)