I bought GDE just prior to a 6,000 mile trip pulling a trailer. The mileage gain has been about 1.5 sometimes 2, nothing to shout about. But the performance difference is significant. I later deleted the dpf and got the modified tune to include that. This shuts off the extra fuel you burn on a "regen."
While on the trip, I put the stock tune back in so I could track the mileage difference. Sorry, I could not leave it in for a whole day or even a whole tank. After driving 2000 miles with GDE pulling the RV in the mountains, the stock tune felt so wimpy and the turbo lag was uncomfortable that I had to stop and put the GDE back in.
Before the tune, my wife used to complain a lot about turbo lag... just a hesitation when you start of from a light, or pull out into traffic from a stop sign. She has not complained even once after the tune. Our acceleration is now smooth and powerful. The lag is way less.
It is a painfully expensive tune.
2007 WK CRD, Quadradrive II, OEM Tow Hitch, GDE EcoTune, DPF-Delete, 4xGuard Front, Rear, Belly Guards, Side Rails & Full Matrix, OEM Skids, EHM, Dash Duck.