Spark plugs & Mileage

[Old Guy]

Gang, we're blessed with a 2011 Carrera GTS. The car came to its 4 year point in April with just 33,600 miles, but wanting to do the right thing to keep my Porsche CPO and extended (from a college frat brother) warranties in effect, this week the car went to Hoehn for a spark plug change. This is due at 40K miles or 4 years. I asked for, and got the removed plugs. They didn't look "that" bad. So today we had to drive up to the San Gabriel Valley - a 200 mile round trip.

We've put 4000+ miles on the car in the last year. Never got better freeway mileage than just over 20 mpg. Today, running typical I-5, 57, 60 speeds: 65 to 80 to some stop/go, guess what?? The car's computer is telling us we got just under 24 mpg! 3+ better than ever in the last year! That's 30-40 more miles for every 10 gallons!? This is the most overt mileage improvement I can recall from any of our many cars over many years, simply from changing plugs. FYI, the air filters are almost new, and never did we use cruise control.

Is this my imagination or are 997.2s and other late model Carreras super sensitive to spark plug changes

[Jad]

I am not an expert on the .2 engine, but I doubt the spark plugs resulted in a 15% + improvement. They are required by law to fill the tires with air (maybe they were low or got overfilled?), there could have been a nice tail wind, the traffic flow could have kept you more at 65 mph and less 80. LOTS of variables that I suspect led to the increase, not just the plugs. If they were that bad, the car would have given a warning light I suspect.

It will be interesting to see what happens over the next 4k miles...

[Old Guy]

For sure I'll be keeping on eye on it. I will mention that I chatted with a well-known garage owner during Sunday's tour who opined that new plugs can make a noticeable difference...whatever that means. We'll see!!

[Jad]

Noticeable, absolutely, but 15%+ increase is too much in my opinion. My 997.1 motor was smoother and revved easier, but no noticeable change in gas mileage.

Hopefully you are right, and you will see the higher mileage for a long time

[pecivil]

for what it is worth, I just did the 4 yr plug change on my Cayman, at 25k miles, and have found a jump from 22.5 to 24.5 mpg (8%) since with no other real factors to account for the increase.
Now I know why! lol.

[Old Guy]

Just to cloud the issue: I just realized that there was another change in variables. We always and only fill up with Chevron gas. It happens that the day our mileage jumped, we found ourselves up in the Inland Empire but almost out of gas. I filled up with Shell hi-test...the only station between where we were and the freeway. Our drive back to Oceanside...with the mileage bump up...was on Shell gas. Just adding this tid bit. Others with opinions about brand of gas and mileage??