Hello,
The rpm's on my wifes Avalon drop like a rock when I put 'er in neut. Coasting @ 65 mph, with 600 rpm.
I tested several vehicles at work this way, practicing my P&G. Engine running.
Everything I drove from a 2006 Mack twin screw tractor to the 2002 Caravan. All had lowered rpm in neut while traveling at highway speed.
Caravan was worst. 900 at speed, 700 stopped.
Mack was best. 550 at speed, 550 stopped.
(All of our trucks are automatic. maintenance costs are lower than with clutches.)
BUT, all were way lower than your 1500 to 1700 at speed.
I will get my hands on a newer Ford product and see what kind of results I get.
S.