Engine temp

[raptorman53]

The other day I was driving down the interstate pulling my 16 foot trailer with two fourby atv's. I saw it was having a hard time in overdrive so I put it in third. When I did this I saw the temp drop almost instantly. I then went back to OD and the temp came up just as fast as it went down. So my question is why is my truck running cooler at 3700 rpm as opposed to 2600 in OD? Anyway I put a few picks up to describe what I'm talking about. You can look at the mileage to see how fast it change. Thanks for any responses and Happy Holidays.