The year they abandoned the CAD (central axle disconnect) Chrysler put out a press release that it was costing 1 MPG

If your tires are really bad rollers,
it is possible that you would get less than 17 MPG on a 60 mph test run.
SIXTY Miles per Hour.
60 mph can be easily checked because ten Interstate mileposts should go by in exactly ten minutes.

For testing purposes you can narrow down whether poor fuel economy is being caused by the engine or the drivetrain/body by coasting down a hill and seing what your "terminal velocity" is by gravity alone.

Many "steep" hills on Interstates leading down to river bridges are 4% grade.

On a long four percent grade hill a 5400 lb 2wd Ram with Bridgestone Deuller HL 245/70R16 tires will coast in neutral at approximately 70 mph on blacktop, and slightly higher on concrete pavement.

A rough way to test engine efficiency is to idle the engine without the AC on for a long enough time to measure how much fuel it uses.

A 1995 Magnum 5.9 V8 idling for three hours will use 2 gallons.

A good condition 2004 5.7 Hemi will probably use less than 2 gallons.