What's the theory on how spark plugs are going to affect either gas mileage or mixture?

Not saying it's a bad idea to change the plugs, it certainly is. If it were easier to do I'd change them all the time. I change them like every 3K miles on my motorcycle. But I bet this is placebo effect. On a modern computer-managed system like this you won't affect fuel mixture or fueling performance with new spark plugs unless the old ones included at least one bad plug which caused a persistent misfire, check engine light flashing, and nuked catalytic converter.

If your Jeep in fact smelled like it was running rich, either it's in your head or there is something else seriously wrong that doesn't get fixed with spark plugs.

I don’t really have a theory, just initial observations. Literally one day’s worth of driving after swapping the plugs. As mentioned in my other post I have roughly 100 miles on them, and it’s a little early to tell, that’s why I said my mileage “seems” to be returning to what it would do a few months ago. (In the last couple of months it has fallen off from 17-18 mpg down to around 15). Since changing he plugs (again one day worth of driving) it is back around 17. Gas mileage is pretty hard to judge on that little mileage, maybe my foot has been a little lighter, maybe my cargo has been a little lighter, maybe there was a good tail wind. Hard to say. That’s why I said “initial impressions”.

As for the rich smell, for the last few months it has smelled a little rich at start up, leaving my house, or work, and getting straight onto the highway 70mph, if I have the windows down I could smell it. I have been told this is due to the cats not being up to temp. Idk. Then, yesterday, after swapping the plugs, it didn’t smell that way. Again, only one days observation.

Yep. Not the plugs though. If the previous spark plugs were working, as in functional, then the fuel ignited, every time. Otherwise you'd get a misfire code and CEL. New plugs, fuel also ignites, every time. It's a myth that somehow "new plugs" means "more spark" and "more spark" means "more power". You just need enough spark to ignite, and less than that means misfire. No misfire before means no difference now. Or at least any difference is something coincidental, and unrelated.

And like I said, unless you had a flashing CEL and a hard/persistent misfire that would fry the catalytic converters in a few minutes, then there was nothing wrong with the previous plugs, besides being old and worn and potentially closer to creating that gross failure condition.

Again, probably not a rich smell, nothing to do with cats (they don't burn fuel, unless you have a misfire, as above, and in that case they are going to be melting rapidly). Not doubting you smell something when backing up with the top down, since you will be immersed in exhaust that way. And there may be something you can smell that would be burnt by the catalytic converters once they are up to temp. But it's not fuel, so it's not a "rich smell".

And, FWIW, it's probably far more likely related to weather change than it is plugs. Like I said, plugs don't somehow make fuel burn more completely or more effectively. They either ignite or not. It's binary. And to not ignite is either an intermittent annoyance (occasional CEL, P030x etc) or it's consistent and an emerging catastrophe.

In other words, new plugs don't improve performance in modern engines. They reduce the risk of failure due to wear, and failure can be very expensive and would be very obvious. If you don't replace them when they need it and you drive while it is misfiring then you will definitely notice a difference when you replace the plugs because the check engine light may go out at least, you won't hear a miss in the idle, or a hesitation intermittently while running, and of course if you nuked the cats then you will replace those along with the plugs and it's an understatement to describe the difference between melted cats and new cats as night and day.

But these old wives tales about fuel economy or incomplete burning of fuel from worn plugs went away with the advent of the catalytic converter and electronic ignition, which was in 1975 for most cars in America. Before that engines would misfire all the time as a normal course of running, you'd never notice it was doing it, and spark was so weak and poorly timed that a slight change in gap or wear on the cathode ("strap") or electrode of a spark plug would screw up the ignition timing and make what was a barely-capable spark suddenly become incapable of igniting the charge. So for your grandpa's '71 Caddy, new plugs, every 10K miles!, would indeed improve fuel economy and performance.

Fair enough.

I am of the belief that the condition of the plugs can noticeably affect performance. I have always noticed a better running engine after installing new plugs. Wires too if the boots rubber was hardening.

The 2007 Owner's Manual has changing the plug wires at 99,000 miles (along with the plugs).

Yes it does. This is Chrysler we're talking about though. Don't trust their recommendations at all. My original plugs and wires were way gone at 70k. No electrode left at all.

Really? Here were mine at 90K.

Attachment 676288

I swapped them there because I was installing a dual-battery tray that would make it more difficult to do the passenger side. LOL, here's what I had to pull the next time I swapped them ...

Attachment 676289

I think your early Jeep might be different from others. Yours seems to be of a better build quality. I have read that the JK 3.8's were not entirely similar to the Dodge Caravan 3.8's in that some lesser quality parts were used for whatever reason. The rod bearings, for example, can float and result in a loss of lubrication since they are more easily prone to shifting. Another example is the piston rings; many claim that they were installed incorrectly. Perhaps Chrysler started to cut corners in 2008 amidst their financial crisis? I wonder if your 2007 3.8 is exactly the same as the tough as nails 3.8's of the Caravan. You don't even burn a drop if oil, if I recall. Interesting. For those of us who consume oil, we'll most certainly experience less spark plug life as the plugs are exposed to oil in the combustion chamber. More heat, occasional if not consistent predetonation, a less clean combustion, carbon deposits, etc.