84 D-350, auto, 4.10's - with a stock 72 440.
Engine has been running cool since March when I started driving the truck. Now that the atmosphere temps have raised here in Tennessee and its officially "hot" out - the engine temp goes up and down with my speed.

45mph - 190
55mph - 210
65mpg - 230

There is a 30-degree difference from top of radiator to bottom; 200 top, 170 bottom. Good mix of antifreeze and distilled water.

Although the mph/temp readings above are from a cheap aftermarket set of gauges, the heat gun on the thermostat housing and radiator are about 5-degrees less - and for what its worth, the dash dummy gauge just starts to go over the mid-way point when its at 230.

Could this be a timing issue that has only reared its head now that its 100-degrees outside?

What say you?

Thanks.