Engine Performance problem
1996 Ford F150 V8 Two Wheel Drive Automatic 98,620 miles
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Looking for better gas mileage: 5.0L Automatic with overdrive trans: Ever since I've tuned up my truck (2 years ago, maybe 5,000 miles) my gas mileage has decreased. I've pulled the old parts out and compared them: Original wires were 8mm Motorcraft -each numbered to match firing order with year 1996 on them. I replaced w/NAPA (Belden Max) Premium 8mm wire set (box & NAPA stated better energy flow & protection index). Plugs pulled out were Autolite 25; I replaced w/Autolite ASF42P. Mileage is averaging 13.39 without A/C on normal city/country with some hwy driving. Highway only w/o A/C on - 17mpg; all utilizing overdrive. Odd, but mileage on front tank seems to be worse than when running on rear tank.

I just discovered wires# 5 & 6 were loomed next to each other (apparently a no, no because of spark over). Wires# 1 & 3 look to be loomed with greater distance as well. Any ideas how to get better mileage? Tires are inflated properly and automatic trans was flushed at Ford Dealership (a special additive was introduced by Ford for shuttering problem).

A year ago, after I tuned this truck up, I was driving through the mountains of TN, check engine light came on. Pulled over at another Ford dealership, they stated there were no specific problems, but they noted my spark was jumping all over the place (could this be because wire#5 & 6 were loomed next to each other). They told me don't worry, drive on. Light has never come on again. Gas mileage & power still stinks. Not ready to drop the cat. converters yet, but I need to get better mileage than this.

1: Any ideas on how to improve my mileage?
2: Should I revert back to the Autolite 25 plugs (thought hotter was better)?
3: Obviously cross spark can reduce gas mileage, but I've never heard of looming spark over issue with #5 & 6, then #1 & 3 before? At night I didn't see any physical evidence of spark over, but have just moved them (just to be safe).
4: Do I need to go back to O.E.M. 8mm Motorcraft wires? Not sure I understand why?
5: Is there something I'm missing to get better mileage?