not answering the very simple questions about sample size sure seems like avoidance

normally i’d ask how you ‘figured ’ that, but based on how thread has gone i don’t think that will be productive.

 

The lie o meter is inaccurate by about 7% optimistic, if it was never corrected.

If you used it before the plug change and are using it now then an increase in observed mpg is valid, but as said would be more accurate on a longer trip, and is best measured tank to tank.
Calculate the mileage after each fill up the old way to find a 100% accurate current value.

You can search here to find threads on accessing engineering mode to correct the lie o meter if you are so inclined.

Not trying to pick the flysh*t out of the pepper, just offering another viewpoint. I'm sure its doing better with the new plugs.

 

I do a rough hand calculation on every tank of gas.. Not very scientific, I just fill the tank till the pump clicks off and use the trip odometer reading. I I average about 18-19 on 87 octane. I did see as low as 16.5 this winter, (a combination of winter gas and 0 mpg while warming up the truck n single digit temps). I think that's pretty good, considering my 2002 Ranger and my 2011 Nissan Frontier averaged about 17mpg despite being a lot smaller and having a lot less power. My driving is almost always over the same 30 miles of two lane paved roads, speed limits between 35-50.

The lie-o-meter is consistently about 1 mpg optimistic. (I say 1 but it could be .9-1.3 or something, as I said, not scientific just observational). I generally do it just to watch out for a dramatic change in mileage.

 

With the ignition timing advance logic in these trucks, I find it hard to believe that changing the plugs would have any effect on daily driving performance even if they are worn out. If the gap was larger and it was delaying the spark, I would think the PCM would see that in the knock sensors and advance the timing, offsetting it.

 

Wider gaps (vs designed spec) mean less consistant burn. Wider gaps are harder to jump especially when you get closer to WOT. Efficiency drops.

Wide gaps create more current draw for the coils, more heat build up. Nothing is saved by not changing plugs regularly, coils are more likely to fail by running hotter.

Resistors in the plugs break down over time and miles sometimes causing a occasional missfires or a secondary gap within the porcelain insulator, (depending on plug design) further reducing spark intensity.

Very minor changes in timing can be had by playing with plug gaps but it is so small that it has very little to do with efficiency vs new correctly gapped plugs.

Tuned engines often require slightly smaller gaps, especially on ecoboost engines because the tune increases boost pressure and fuel delivery. Higher cylinder pressure "insulates" the gap at the spark plug making it harder to jump, reducing the gap restores its ability to "push through" the increased air and fuel molecules allowing for consistant operation to be restored.

 

This all makes sense from a performance standpoint but this is an MPG thread for someone gently daily driving a truck. The RPM's and cylinder pressures are likely not very high and the coils are not under high stress to be lighting the spark off at 6000 rpm.

 

If the ignition system is compromised, its inefficient in all driving conditions. Coils run hotter no matter how its driven, coil boots degrade, once missfires get bad enough the cats begin to fail.

I've always experienced increased mpgs, or maintained good efficiency, and a smoother running engine after changing worn plugs.

I usually dont let them get as bad as some do but I do believe that replacing worn plugs to will restore/improve mpgs, even on daily driven lightly used vehicles.

As long as cars have been on the road spark plug/ignition system maintenance and general engine maintenance has been a requirement to maintain efficiency and reliability.

Risking having a plug sieze in the head and ripping out the threads or breaking off when trying to remove it is incentive enough not to ignore plug maintenance.

 

Worn plugs have sharp points or edges from erosion and have lost mass and surface area. I think they can glow red under certain conditions. Perhaps the knock sensor detects this and retards timing a touch.

 

This is what I've seen consistently in the 24,000 miles on my '25 3.5EB. 1.0 mpg, +/- 0.1 in local, highway and mixed driving.

 

Funny enough, wide gaps actually increase efficiency IF the ignition is capable. It exposes more of the flame kernel. I have a 1997 S10 4.3L that was factory gapped at 0.060". Thing used to rattle like a can of marbles uphill in fifth on regular. I used to buy premium at 88 cents a gallon in 1998. Later models dropped the gap.