I think I tried those SplitFire plugs like 15 years ago drawn in by their 'story'. My mechanic at the time pulled them out months after install and showed me how a spark plug should look like after a year in a car under similar conditions to my own. There was a stark difference between the two plugs.
I've tried a bunch of supposed tweeks for my vehicles and few have ever born out in benefits. Not to say someone won't one day make some technological leap, but I don't see it.
These are what worked and is done in minutes, in whole or part, for me:
Upgraded plug wires - Slightly more power felt, no mileage benefits noted. That was 15+ years ago, since then I've driven (distributorless) SAAB's
TUFOIL - Remarkably less engine operating temps ('92 Volkswagen), but increasingly less benefit that way every car since. I think engine operating temps and (combustion) friction has likely been reduced in all cars over the years, mitigating the positive effect to this to very little now. There may have been small mileage benefits initially, but I haven't noticed more recently
Marvel Mystery Oil - Crazy stuff works in every car tried (in gas tank). Smoothest running engine always on MMO - even wife notices. When my main engine seals went on my old SAAB at 12 years and 90,000 miles....my long time SAAB mechanic said you could eat off the innards of my engine.....he had never seen any motor look like that at 90,000 miles. Mileage benefits seem to be on the order of 2-3% consistently thru the years
Engine Tuning - At least with turbo cars, and dependent on maker, really kicks max HP, torque up; generally makes higher torque at much lower engine RPM's, reduces turbo lag a lot and, I've found, mileage benefit of ~ 4%. I don't stomp my cars, and I suspect I drive the same speed(s) now, but the accelerator is depressed a little less to reach the same speeds now....so some mpg is spared. An unexpected benefit to a pricey ($1000) upgrade that I expected only performance benefits.
I've read the re-mapped CPU doesn't do much of a thing for non-turbo cars, however.
Oh, and btw, I've had the pleasure of sitting in and driving my business partners TESLA S (with performance plus pack) a couple months ago at the factory in Northern California and I don't think a car with internal combustion motor is in many of our far off futures anymore. It accelerates and handles like a Porsche (0-60 in 4.2 seconds, center of gravity 6" off the ground, etc); and cruises as quietly as a bicycle. The WHOLE car is an internet hot spot and you have wi-fi access 100% of the time thru the 17" touch screen.
I've been reading all these comments on the 'net by folks that have not been in one and that they would buy a similarly priced Porsche/Audi/Mercedes/whatever instead......and I call 'bs'. The TESLA is far and away the finest and most remarkable car ever made in my lifetime. Right now it's unobtainably priced for most at $80,000-$100,000 (with government rebates of $7500), but as that technology ripens it'll make future talk about spark plugs meaningless.
His version goes 250miles between fill-ups - the recharging is either free (at TESLA superchanger stations) or nearly so (at home, etc). If battery technology gets a little better or TESLA starts offering hot-swapped, fully charged battery service (it's coming)...they'll be little reason to consider anything else at the higher price points for cars.
I'm saving up my pennies (my partner tends to spend more freely than me and doesn't have a kid)
