Hot at highway speeds

Well, the GV reduces your engine rpm to 78 %. So working the math the rpm with the GV musta bin 3000 x .78 = 2340.. Now, going the other way ; 3000/2340=1.28, or a 28% increase in revoutions per minute. Your engine is firing 28% more times per minute. And every extra time it fires it creates an extra bit of heat .Thats part 1.
As the rpm goes up, so does internal friction.Thats more heat.Part2
The oil pump is working harder and the crank is beating up the oil ; More heat. Part3
The higher crank speed is spinning up the TC, which spins up the transmission oilpump which just pumps the oil around in a circle and dumps it back into the pan. More heat going to the rad cooler.PT4
So where does all this heat go? Well,some is re-radiated through the iron walls of all the engine bits, and some out the tranny case, and some goes out the exhaust. But a great deal of this extra heat ends up in the rad.
And when the rad cant get rid of it fast enough, the coolant temp starts to rise.Eventually the rising liquid temp finds a new equilibrium point at which the rad can shed the heat fast enough,if the engine lives through it.