2003 Mazda Tribute idle

Good day,

JUST IMAGINE

now, I am driving my car at 65 miles/hr. Down hill.
my rpm shows around 1900-2000. To save some fuel I change the auto. Gear to N (neutral). The rpm goes up to 3000 some revs. Why.
my other cars' rpm falls down to normal idling ratio of 800-1000. Under the above driving conditions.

no mechanic answered this question. They could not figure out.I hope you reason this case somehow.
my best and regards.
matt ugell