Draco713:
Just to expound/clarify on adsm08 comments:
2001 and newer Rangers get their raw speed signal from the OSS (Output Shaft Speed) sensor on the transmission/transfer case. So, both axle ratio and tire diameter matter when calibrating the speedometer/odometer.
So, you need a combination of gear ratio and tire for the Ford tech to program.
e.g. Your '10 Ranger might have come with 3.55:1 gears and the 235/75R15s; but 245/75R16s never came with 3.55s (They usually came with 4.10:1s)...
Which is why the 1st shop can't adjust it correct (nor would any subsequent).
As others have noted; you need to get a tuner to adjust it correctly.
Now, back to the thread jack:
Ford milked the Ranger to the point where regulations were requiring major changes (anti skid, crash structure, etc, etc). Given the crisis at time; Ford couldn't commit to every program and Ranger was one which died on vine.
Building a '15 Ranger with a 1.6l Eco boost (same power as 2.5l Duratec), 6 speed auto (from "world Ranger"), stupid high rear end gears (2.73:1 with the 6 spd auto would be same low ratio as 3.73s were with 5R55w), high strength steel frame/aluminum body (baby F-150) would result in Ranger having fuel economy in 30s; but it would cost almost as much as an F-150 (smaller volumes, so fewer units to amortize engineering/tools costs over)...