What do you think about these temps and why?

[old31]

I installed a Rex a Co temp gauge and this is what I have.

With 80-90 degree outside temps I can idle all day and stay at 185-190. Drive around town all day and stay at 185-190.

This is what I have noticed twice. On highway trips of maybe 30 miles at 50-55 mph when the outside temp is about 70 the car temp will go to 210.

But on the same trip when the outside temp gets to 80-90 the car temp will rise to 225. It does not boil over. But I can slowly watch it creep up on the highway.

I am not sure of the age of the radiator but am guessing it is 7 years old.

I am running 50/50 mix, 160 thermostat, engine pans, leak less pump, 4 blade fan, Snyder 5.5 head, a newly rebuilt engine with 1,000 miles on it.

So the question. What is making the temp get to high at fast highway speeds when the outside air temp increases.

Is it worth trying some HF floor degreaser or 1 1/2 cups of dish detergent, or Simple Green, just to try an easy something?