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On the highway, the faster you go, the higher aerodynamic drag you have. From my observation, coasting in a deep long downhill, coasting in neutral will not accelerate forever. The speed will be stable in certain point. Same to coasting in gear, but the speed will be lower. The speed gets stable because of the external drag and internal drag. External drag is aerodynamic drag and rolling friction. since coasting in neutral runs faster, higher aerodynamic drag. Internal drag is engine braking force, in this case, which coasting in gear is bigger.
From my daily commuting, coasting in neutral on a particular downhill is 70 to 75mph, depends on traffic (drafting effect) and which lane (different slop angle since the road is curvy) Coasting in gear is 60mph max. on that road.
How far you can continue coasting after downhill depends on 1) the speed the car is running at the end of the hill 2) the drafting force the car has. That is why coasting in neutral goes further because of those advantages.
If, the car is coasting in gear (fuel cut) on the first half, then coasting in neutral on the second half, the car will coast similar distance as all the way coasting in neutral, but save gas on the first half of the hill. Is this strategy right?
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