You may be able to adjust the shift points on your Rangers Transmission.
I owned a 2005 Ranger 3L auto a couple years ago and looked into it.
It can be done thru the ecu on my truck, I remember reading that I could change it to kick in 500 rpm sooner but it may cause the tranny to have a bit of a shimmy when changing into od. My Ranger was a gas pig it was as if it was De-tuned to be as inefficient as possible.
My old 92 Cherokee with its 4L motor {5-speed} and 4" lift kit got the same mileage as the 05 3L Ranger{auto}about 20 mpg..imperial