I've done some experiments with 8/3 cable and 40 Amps. There is a noticeable, but acceptable temperature rise when the cable is laid out in an open pattern on the garage floor. The problem comes if you leave the cable coiled up. In that configuration, the runs of cable lay next to each other, and heat each other up. I did not do any calibrated measurements, but I considered the temperature rise unacceptable and dangerous with several loops in a coil.
If you do use 8/3 to carry 40 Amps, I strongly recommend that you use care and unroll coils completely. Also, check the cable and connections for unacceptable temperature rise, after connection, at 5, 15, and 30 minutes.