Besides not having a lead foot, I've been reading people getting better economy with mods. New headers that gets rid of the u bend, intake, gen v. Do tuned pcms ruin your mpg? What's it take?

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2000 Silver Grand Prix 4 Dr GTP 180k miles
Speed Daddy Headers
Gen V
180 Thermostat
HAI
Aeroforce Scan Gauge

My mileage is better than stock because of my tune. Stock, my LTFT's were -21 which is super rich. After I tuned it, my LTFT'S are right at 0 where they are supposed to be.
I was lucky to get an average of 22-23 mpg stock, now modded and tuned I get an average of 27-28.
I tuned my car specifically to it's needs, a generic PCM may or may not make a difference because every car is different.

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3.5" MPS, 1.84 Rockers, 3" DP, Frt PEM, 39lb Injs., PowrTuner, Wizair
14.07@ 98.85mph 2.180 60'

He is right. But, there is more to it than that. As you make the engine breathe better, and tune the PCM for it, it becomes more efficient. That naturally means more power, and done smartly, better fuel mileage. My L67 Olds made about 310hp and returned 35mpg on the highway at 75mph with the windows down since the a/c didn't work. I know there was another 15hp or so in it with that hardware, and I was working on even better highway mileage with good results.

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One Big Ass Mistake America

Making the engine more efficient in general makes it net better MPG's. Remember it's just a huge air pump, thus is you make it easier for air to get in, air to get out, and make sure the bang in the middle reaches it's full potential, you'll see gains. So any sort of breathing mods, definitely including headers on the S/Ced engines, and a more efficient blower like the Gen V which produces less heat than the GenIII, along with a good CUSTOM tune will get you better mileage over stock.

Box tuned PCM's can sometimes ruin MPG's due to them running on the "safe" rich side, thus a custom one would get you better AFR's and LTFT's.
The lady friend's 97 actually get's 33-35MPG on the highway and runs -10 LTFT's right now on it's current custom tune. Not too bad for a very slightly modded N/A engine

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

I have a question regarding - fuel trims. Is it normal to have - trims around -10 while cruising or would it be best to be at 0 while cruising? It locks in to 0 at WOT and so does the stft. the stft fluctuates between 0 and -3 while cruising...

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It's best to have LTFT's be zero.

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

Ok thanks, that was a fast response! Im thinkin the -ltft's are coming from my tune now since I have eliminated all my exhaust leaks..

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You still running the ZZP box tune? And yes STFT at WOT should be zero as to not mess up the LTFT number (since PE mode causes a rich condition) which basically acts as a correction factor for the PCM's fuel quantity. STFT fluctuation is obviously completely normal though as long as it's not going above +-10% and if it does go above those boundaries for more than a set amount of time, then LTFT's are taken into account.

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Dubs:00 VW GTI VR6, Low and Slow 91 VW GTI 8v, Shave the Bay!! 91 VW Cabriolet 8v, bitch Basket
GP's:04 GP GT, Past now G-Ma's Girlfriend's 97 GP GT, "Project Pink Power"

Yup still running the ZZP 1.0... Don't think i've ever seen the stft above +-3%, so I guess that the LTFT's would be ignored then... Hoping to get into tuning soon but might get a tune from will overkill in the mean time.

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