Finally, an update worth noting.

With a new thermostat that actually opens, I've been getting much better results. I haven't seen anything over 40% from the fan driving through town now. With the hood open and the fan at that speed, it's barely audible At idle it hasn't needed much more than 25% from the fan.

And it looks like I'll have to add logic to turn the fan off completely if the temps drop too far below the target temperature. At highway speeds, even with the fan at 25%, I'm not seeing the temp go up drastically at all from interference from the fan.