I am stock except for a turbo back exhaust and I have pulled a 4000 lb trailer up some passes in the mountains (and not the real big mountains) and got over 1250 and have seen almost 1300. I let off and or let it down shift and slow down some and proceded on. I wouldn't have thought it and was glad I had a pyro to see it. With the exhaust, I would have thought I wouldn't have come close to the temps I have stock but did. Go figure....