The computer controlled electronic fuel injection system maximizes fuel economy. If you're coasting downhill, regardless of engine speed, with the throttle closed, fuel consumption is minimal. Fuel consumption occurs when fuel is sprayed through the injectors. Injector "pulse width" (how long they are open) is controlled (in addition to engine speed) by temperature, throttle position and engine load (intake manifold vacuum). To go one step further, with the throttle closed, manifold vacuum is high, so fuel consumption would be low. At larger throttle openings, vacuum drops, fuel consumption increases. At highway speeds, the lockup torque converter engages to improve fuel economy by eliminating drive line inefficiency. That is why the engine speed (RPM) is constant coasting downhill at 85 mph.