Bill's tunes can be tailored for "efficiency". I got better mileage using the canned 87 tow tune for non-tow driving compared to the stock tune that was built into the truck - but I can't give you "hard" figures, it was too long ago and I did not use the stock tune for very long at all. Bill's tunes improved on the canned tunes by about 0.5 mpg.
However, there are no "guarantees", here. I can only talk about my own experience.
- Jack
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