Keep in mind the "average MPG" includes idling (0 MPG).
So, .038 is within spec.
Plugs should last at least 100,000 miles, that is the recommended time to change them. The maintenance schedule, and gapping is in your Owner Manual (link in my sig).

Since putting these in, I've already put in 12 driving cycles, about 70 miles, with no missfires. I'll be doing a bit of running around today on various errands, and will finish off with a 60 mile trip on the Interstate. Should end up being around 125 to 140 miles total today.

Did you ask the dealer to replace the spark packs? They usually last forever, not a maintenance item. Look in your Owner Manual for the maintenance schedule; 100,000 miles is plugs and filters and looking at the exhaust and fuel system and trans fluid (if used in severe service).

LOL, no such thing as "forever" coil packs, they do wear out and are the primary cause of misfires which all 4 cylinders had shown.

Not this century. Most "misfires" are not really misfires they are caused by a loose timing chain.

Zero misfires in the last 427 miles since changing plugs & coil packs. I was getting infrequent misfires almost every drive before that, according to my old Craftsman OBD2 reader. I think that's a good indication it was plugs and/or coil packs...

:funnypost:Not even close.... at least according to: the below google search results, three different Chevy dealers, and my independent mechanic. All 4 shops agree on the top 4 in this list (not necessarily in order), they vary a bit with the rest (usually only naming about 6 or 7 causes), but timing chain is consistently near the bottom or not even on their lists, also none of the shops said fuel pump....

1. Worn Spark Plugs: Worn-out or fouled spark plugs, especially after long-term use, are the most common cause.
2. Ignition Coil Failure: The coil-on-plug packs frequently fail on the HHR, leading to a loss of spark.
3. Vacuum Leaks: Leaks in the intake manifold or cracked hoses introduce too much air, causing a lean misfire.
4. Faulty Fuel Injectors: Clogged or damaged injectors, particularly in cylinders 3 or 4, often fail to deliver fuel.
5. Carbon Tracking: Oil or moisture causes carbon paths to form on the spark plug insulator, grounding the spark.
6. Failed/Failing Fuel Pump: Low fuel pressure from a dying pump fails to sustain combustion.
7. Timing Chain Issues: A stretched or slipped timing chain causes poor valve/ignition timing.
8. Mass Airflow Sensor (MAF) Failure: A dirty or failed MAF sensor disrupts the air-fuel mixture.
9. Camshaft Position Sensor Failure: Faulty sensors cause incorrect valve timing.
10. Cylinder Head/Valve Issues: Burnt valves or broken valve springs lead to loss of compression

BTW... there's no such thing as "not really a misfire" either the cylinder fires when it is supposed to, or it doesn't.... if it doesn't, that IS a misfire, regardless of the cause of that misfire.

A member here a few years back didn’t check his timing chain, threw parts at his P0300 and P0303 codes!
the front guide was damaged and he blew his engine when the timing chain jumped and the valves damaged the pistons.

Been on this site since 2009, your info is wrong or generic or old. You need to understand what the computer sees as a P0300 "misfire": a variation in the speed of the crankshaft, NOT an actual misfiring of a spark plug. P030x is a different thing: the computer figures out which cylinder is misbehaving by telling what cylinder is supposed to be firing when the variation occurs.
Before OBD2 you could tell a misfire by listening to the engine, because there was an real misfire of a spark plug.
For some reason P0300 gets the name "random misfire" or somesuch. What it actually is :"misfire detected" and misfire does not mean what an old time mechanic (before VVT) thinks.

That list is a specific list for Chevy HHR's. Sorry donbrew, but I'm going to trust an ASE Master Technician (my dad's best friend of 40+ years) over a name on an internet forum, and over Google 1,000,000 out 1,000,000 times, even though he retired 2 years ago - his son now runs the shop. He said that most of the misfires he's seen in HHR's at his shop have been either plugs, coil packs, or combination of both.... with wiring, and injectors being most of the rest.

I've not seen a P0300 in this car with either the Craftsman OBD2 reader, or the new scantool. I've already posted the P030x misfires recorded on each specific cylinder, though those were not the only P030x misfires recorded, just the ones from the last 10 trips, I've had P030x misfires recorded since I brought the LS home on Feb 17th but not P0300.

AGAIN: Zero misfires recorded in the last 427 miles since changing plugs & coil packs. That's zero P0300's and zero P030x's.

and BTW this whole tangent is not directly related to the thread... which is about the NGK Ruthenium HK plugs, and NGK U5180 coil packs. Even without misfires, I would have still changed them out just because of the mileage on them. Kind of like changing your oil before the DIC shows it's due, or having a new timing chain & related items installed before it's due.

And on that original topic, I know it's highly suggestive, but it seems like the engine is running smoother after the swap. Also wrote down the mileage before fill up (mid point of trip), so I can track real MPG manually!

Drive on! I’ve been a mechanic since 1973 , been working on HHR’s since 2007. One thing I ve learned is , it’s your car do what you want when you want to!
Google or Facebook don’t work on cars just opinions.

I’ll take care of my HHRS as I have for a total of 843,000 miles.