The thing is, my JVC headunit also does the volume adjustments. And the difference is that it really look at the car's speed, but the rpm of the engine. My car makes most noise as I am accelerating or going uphill, and gpsapp won't know how hard the engine is working in such situations.

Tire/road noise on a highway isn't that much. Besides you typically maintain the same speed for a while on a highway and adjusting the volume by hand really isn't that hard. Most noise is introduced when someone opens a window which neither speed nor rpm based volume adjustment will ever figure out.