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Wow this is a huge thread, not sure if anyone mentioned this but wanted to toss it in there. I have a 2008 Subaru Forester (2.5X Premium, non-turbo, 5-speed manual transmission) and recently did an experiment where I replaced my plugs with NGK's V-Power coppers but overgapped them to .050" (stock gap was .043")
Background:
0-60K miles - Commuting ~90 miles a day mostly freeway, tended to get around 26-28mpg consistently. Spark plugs changed at 30K by a reputable dealer though I don't know what they used, but the car worked fine.
60K miles - Got the 60K mile service done by a new dealer (I moved), fuel economy instantly dropped to around 24-26mpg. The dealer said "there is no fuel filter to replace on this so we did a fuel system flush procedure", later on I pulled the plugs and they were NGK Platinum's (OEM recommends Copper, from what I gather). Don't know what the gap was.
90K miles - New job, new commute has me going around 50 miles a day mostly hilly terrain with some flat stretches and some bends. Before doing the 90K mile service (MYSELF this time...) I was getting consistently 24-25mpg. After changing the plugs to NGK V-Power coppers, still 24-25mpg.
Around 94K miles - Decided to tinker with spark plug gaps. Pulled the plugs and checked them--the V-Powers were gapped around .044", close enough to the shop manual's gap (.043"). I overgapped it to .050" and reinstalled. Still running the stock ignition coils and ignition wires, never changed them (might be a good idea soon...)
Instant bump to ~26-27mpg with my same hilly 50 mile/day commute. This is over 4 tank fillups so far, with one datapoint in the upper 25's (that cycle I recall a lot of idling and getting stuck behind school busses...)
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