People's experiences vary. My economy improved with the Gotts mod, and, it improved with an Edge and then got a bit better with custom tunes.
One thing, you'll be able to use the "canned tune" Gryphon on BOTH trucks, as long as you return one to stock before moving it to another. So, you'll be able to see what it does for both of them. Once you get custom tunes, it becomes "married" to one vehicle and you lose that choice.
I'd say it's definitely time to change plugs if they have over 100K on them. Use Motorcraft plugs.
If you don't tow, just get an 87 performance custom tune when you make the choice of truck to use it on. Until you get custom tunes, use the Level 2 tow tune as a daily driver. It's a good all-around tune.
If the tires you put on the older truck are larger than OEM, they will probably hurt your mileage. They increase the load on your engine both during acceleration and at cruise, even if the cruising rpms are lower.
- Jack
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