re: F-150 Question

Carbon build up in combustion chamber is frequently the culprit. Of course you should change the plugs @100k and ensure a clean OEM air filter. Assuming no codes.

But some discount gasolines don't have the additives to keep the valves and comubstion chamber clear of carbon. This buildup decreased the effective size of the combustion chamber thus increasing the compression ratio. Also when this carbon gets hot it can glow like a glow plug. SO the effect spark knock. When the knock sensor detects these knocks it retards the timing reducing performance and fuel mileage.

So the cure is to have a combustion chamber cleaning service done by a COMPETENT shop. You may be able to get some improvement by running Tier 1 fuel with a lot of cleaning additives. Chevron, Exxon/Mobile, Shell, etc. Run it like you stole it. It may take several tank fulls.

I have a 2011 5.0 F150 that I use only Shell and Chevron and at 73,000 miles it runs as it did when new.