Vehicle:  2001 Discovery 38D, CAT 330 diesel, Freightliner chassis, Cummins 4300 electronic transmission; odometer: 90,009 miles.

Issue:  My speedometer indicates I'm traveling about 65-70 mph, but my Garmin indicates 48-52 mph.  

Situation:  All seemed fine for the first 75 miles or so.  Over the course of the next 10 miles or so, I sensed that the RV was traveling slower than the speedometer was registering, which was 65 mph. I engaged my Garmin's speed indicator screen, which indicated I was traveling at 52 mph. Being a rookie RV'er, I wasn't sure which one was more accurate, the RV or the Garmin. 

As I was exiting I-16 EB onto I-95 SB near Savannah GA, I observed a DOT work-zone signal that read "YOUR SPEED: 35 MPH" (major re-construction of interchange in progress).  That was exactly the speed my Garmin was indicating, with the RV's speedometer showing 46-47 mph.

It doesn't appear to be a transmission issue, or the electronic shifter, but that is the diagnosis of a rank beginner when it comes to diesel engines (that being myself).

Can't live with that speed, can't shoot it.  

Any words of wisdom, advice, or suggestions?