Add a Vehicle Speed Sensor (VSS) to your EBL and then you can enable Lean Cruise and Deceleration fuel cutoff (DFCO). Then you can watch the Instant MPG goto ~14MPG on lean Cruise and 99MPG going down a hill with DFCO.

I usually run with the truckers here in Canada. So in Ontario on the freeways they are required to run speed limiters at 105Kph max. I've seen 12MPG average (US Gallons) on the EBL (which calculated pretty close on 2 fill-ups). This was only highway running at a constant speed.