only the straightest and slightest of declines are "consistent." Usually going downhill you have to brake around turns (or not pedal anyway). You then accelerate out of the turns. Then it gets steeper and you speed up. Then it levels out and it's really hard to maintain that 38mph, then,then,then,
Forget holding a consistent wattage -- try to hold a consistent speed. It's hard enough when going 30+ for awhile.