I live in Panama City Fl. My faithful 1997 T100 got badly damaged during hurricane Michael and I decided to purchase a used Tundra instead of repairing my T100. The tundra does not have the tow package, but does have an aftermarket hitch. Temp is fine while driving around town. Doesn't matter if A/C is on or not. If I drive for 20 to 30 minutes at highway speed 55-70 temp will rise to about 3/4 or slightly higher on the gauge. It has not climbed into the red yet. If I raise the rpms or downshift I can watch the temp gauge drop to normal range in less than 10 seconds. I have replaced the thermostat, water pump, idler and tensioner pulleys with Toyota parts. Toyota antifreeze. Replaced serpentine belt (Gates) Replaced radiator (Auto Zone) radiator cap (Toyota) None of this has changed the temp at all. I also have done, on two separate occasions a combustion gas test on the antifreeze, at the radiator fill neck, and also on the overflow with radiator cap installed. Engine revved 2500-3500 for approx. 2 minutes. The test fluid did not change color. Out of curiosity I put the tester near the exhaust and it changed color immediately. I took it to the Toyota dealership and they could not get it to duplicate my problem. I pick it up and it does it for me first drive. They are not certain what could be my problem. The Tundra has 226,000 miles on it, but it is a Toyota, and of course the previous owner had no problems with it. I am the 3rd owner. I would love a new Tundra but can't afford it. I am at a loss. Any other ideas?