I stand by what YOU wrote. Do you need time to retrieve your records, your copious notes?
How much is “roughly 8-10”? Is there a number for that?
Tell us again why you disconnected the battery?
If your Pathfinder is 4WD, then it’s rated for 14-17 MPG. So 14-15 MPG is in the ballpark for what you should be getting anyway.
“This is the first time I’ve heard that some engines don’t take well to Iridium plugs.”
We don’t know that for sure yet, but the results you get when you switch back will add to our body of knowledge.
Might want to get a cap and rotor from Nissan too.
p.s. Check the base ignition timing if possible and the vacuum advance diaphragm if it has one.
Might be time to replace the distributor. Could have been so frail that changing the rotor button caused other damage.
You didn’t mention a new air filter. How old is the current one in your truck? Check out a new air filter. Perhaps the duct work to the airbox is somehow disrupted in the process of changing out parts. Your problem with mpg might not be related to your ignition parts at all.
Can you manually check and change the timing of the truck? Perhaps the new parts have retarded the timing slightly. Most cars now have timing controlled by the computer and this a setting you can’t mess with.
Could there be a difference in the new cap and rotor? Perhaps it isn’t an exact match to the old cap?
Spark plug gap can also be a factor. Most Iridium plugs aren’t easy to regap due to the hardness of the materials. Perhaps the new plugs gap is different?
The new plug wires could have different resistance values. Are you certain that the resistance of the old plugs matches the new plugs?
Spark Plug Gap and Tire pressure have more effect on MPG per vehicle than anything else.
Maybe your tire{s} is getting flatter.
" How much is “roughly 8-10”? Is there a number for that? "
Yeah, 8-10. If he's measured the gallons he's putting in the tank, and the mileage he's travelled since last fillup, then he has MPG figures. Why would they have to be written down?
Regrading battery, this is recommended by some after anything more than changing the air filter; if the computer had adjusted for worn spark plugs, plugged air filter, etc., it will have poor fuel trim etc. values recorded, and pulling the battery would have reset to factory defaults which are likely closer to correct. That was 4 months ago though…
I like the idea of checking the air ducting for leaks. That SHOULD result in a check engine light, but who knows?
Secondly, the iridium plugs -- I know in some GM vehicles, they really say they don't like them at all. Why? I don't know, but some people have misfires, and others have them seem to run nice and smooth but lose power or mileage (does this sound familiar? 8-) One person looked into it and found in their case, the plugs ran enough hotter to cause ping or knock, so since they had a knock sensor the timing was being dialed back quite a bit. I do realize a Pathfinder is a Nissan 8-). But, your Pathfinder should also have a knock sensor, and perhaps it is not liking the iridium plugs either.I was racing to the end, and oldwrench beat me to it. 
Spark plug gap is critical. Some say “pre-gapped”. Don`t trust it.
When you change the plugs, check the gap yourself and adjust. There should be a label under the hood or a spec in your owners manual to tell you the proper gap.
I have two suggestions:
Are you sure you’re checking mileage properly? I know that sounds stupid, but a lot of people have no idea how to do it. Each time you fill up, reset the trip odometer. Fill up again, divide the miles on the trip odometer by the gallons pumped in. Doesn’t count if it isn’t a fill up each time.
Secondly, don’t do anything yet. If your car has an OBD II port (it would for sure if it’s a 2000, but might not if it’s a 1999.5) you need to let the engine adjust to the new parts. Your OBD II goes into closed loop mode once it reaches a certain temperature (185 deg. on the coolant sensor). In this mode, it fine tunes your engine parameters according to a “map” until it hits the sweet spot. This adjustment can take quite some time. It had adjusted the parameters over time with your old parts, and now it has new data coming in to adjust to. It may have been a long way from its default settings. Taking the battery cable off for 6-30 minutes (depends on the car) would reset the computer to the default codes, which aren’t necessarily at the sweet spot, so that may help, or it may send you backwards. Either way, wait for a couple of tankfulls before you do anything. Your mileage may return by itself.
You can buy a device called a ScanGuage II for about $160 (worth every penny) that will plug into your OBD II port and monitor your engine sensors continuously, including fuel consumption. It will also return error codes and allow you to reset them. Not only will this point you in the right direction on troubleshooting, it can save you a tow if your engine drops into Limp Home mode because of a transitory signal.
Remember this, though: The error code tells you what sensor is reporting the non-standard condition. It does not mean that is the problem. For example, if your crank sensor sends an error code, it doesn’t mean the sensor is necessarily bad. If your oxygen sensor reports an error, don’t automatically replace that sensor. Reset the sensor and see what happens next. OBD II reports conditions that are outside the EPA standards, so your engine light may very well come on even though the car is running fine, so don’t rush to the repairman until you’ve run down the codes as long as the engine is running fine. I use the ScanGuage II to tell me if any parameters are drifting. I also check mileage with every fill up since it is the surest sign that something is going wrong. Of course, I have to factor in the type of driving I did on the last tank of gas, but it doesn’t take long to get to know your vehicle.
i run a SBC 350 in my 93 s-10 its an old motor and i have never put anything but ac delco STOCK plugs in it and it still runs low 10’s the high performance plugs are a gimmick to get you to spend more cash
btw most irridium plugs are not made in the USA
“One person looked into it and found in their case, the plugs ran enough hotter to cause ping or knock, so since they had a knock sensor the timing was being dialed back quite a bit.”
This makes great sense, IMHO.
Interesting. I have the same issue and wondered if others have experienced this, so I googled “gas mileage worse after tuneup” and found your post. I have a 2006 Acura TXS and it used to get 27-28 mpg on the highway. Over the last year I’ve noticed the highway mpg has dropped to around 25. Two weeks ago I took it to Firestone to get the tires rotated, ended up getting two new tires and a tuneup, including new spark plugs, new air filters (engine and cabin), and new oil & filter. The guy told me that Acura recommends (iridium?) spark plugs that cost $30 per plug. I said ok and I expected my mpg to go back up. It didn’t. Before the tuneup my overall mpg was 24.5. Now it’s 24.1 mpg and it seems to be dropping. Very strange.
I’d be inclined to go back to the recommended platinum tip repolacement plugs. Irridium is harder, eroding less and maintaining a strong spark longer, but the tips is different. The center electrode is a much smaller diameter. That should in theory produce a narrower but more intense arc, but perhaps that motor simply prefers the other arc. I know, it sounds a bit nutty, but just as the spray pattern from an injector can affect engine operation, perhaps on this engine the arc intensity and profile can affect engine operation. You’ve got nothing to lose by trying to go back to the platinum tips and see what happens.
The general rule here is that if you change some paramter and the mileage drops, try changing back. In this case it sounds like that changed parameter is the plug type.
Let us know how you make out.
PostScript: we’ve all done it, but plugs should not be removed and reinstalled. The crush washer on flat seat plugs (some are conical seats) is designed to crush and fill the gaps when installed, and will not recrush properly to new surface imperfections once used.
I have two comments:
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I don’t know what wires you used, but many of the fancy looking wires look great, but don’t function well.
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If you are using an on-board computer to compute mileage, it may have reset with the power out and it may need a few fill-ups to get back to where it was. It may by using default values.