As Ross stated above, the crazy numbers displayed by the DIC when simple handheld tuners are used, do not portray accurate mpg numbers.
The DIC routine is reliant on the original stock fueling table, to remain accurate. All the handheld tuner manufacturers, change the fueling table. Some change just the WOT areas of the table, some change all sections. The ones that change just the WOT sections, can retain some DIC accuracy at part load/part throttle settings.
But for the most part, the manufacturers always lose DIC accuracy, and are not concerned, because the customer is fooled into thinking he has gotten a better deal than he really has. Smoke and mirrors by ommission. They don't tell you that you will get an 8 mpg improvement, they just let you discover it for yourself. You (unwittingly) become the manufacturers best sales tool, as exemplified in Post 4
. Its a shame.
Economy benefits are possible with handhelds, but you will have to hand calculate the benefit. The DIC is unreliable. In my latest offering, I get 39-40 mpg on the highway (only because I created an all new fueling table). If I didn't change the DIC calibration table it would be a "miracle tune". I use a spreadsheet to correct the DIC, but EFILive is the only product that allows me to do this. Still, I can get a solid 22 mpg on the highway, not bad for an 8000 lb beast.
Don't be fooled, most handhelds provide little or no economy advantage, they all overboost at no-load fueling, and over retard timing for no load conditions.