Thread: More fuel economy woes.. 2004 2005 GTP

Any (relevant) thoughts welcomed! I currently have an 05 GTP with the following mods: 3.5", AL 104's, CAI, ZZP stainless downpipe/hi-flo cat, 180 Tstat, 1.8 Yella Terra Full Rollers, ZZP PCM 1.0 (with their default, custom tune... this was all ordered from them at the same time, so their tune is supposed to have taken these mods into consideration). All of that being said, I had the same poor fuel economy--for all intents and purposes, identical--on my bone stock 2004 GTP and my current 2005 GTP before mods.

I ALWAYS run 93 octane. My mods have made virtually no difference in mileage (but a damn sight difference in power when the foot goes all the way down!).

I have tried all variations of driving styles (had the 04 from @2007-2011, and the 05 from @2008-present). Short of keeping my foot on the floor at all times, there is only a slight difference in average economy between being very consciously light on the throttle and what I would consider "normal" tendancies--ie: not grannying from stop lights, but not gunning it. Usually just beside or slightly ahead of other cars leaving lights.

I NEVER average more than 20MPG, with a fair mix of city/hwy, 40%/60% to 30%/70% respectively on "normal" tanks (no long trips). I have city driving 5-10 miles at a time, and a highway commute of around 40 miles, round trip. On long trips--where I would stay on the highway throughout an entire tank, stopping only to refill--the BEST long trip I have ever averaged is 25, with long trips usually being in the 22-23 range. Average driving with a 1/2 tank or so trip yields a best of ~21mpg.

All of these numbers are from actually calculating every fill up. I track every fill up with a Fuel Log app on my phone. The DIC is consistent, but generally off on the average by about 1.5mpg. At 60-ish MPH highway speed, I will see the instant economy reading in the 30MPG range, but that has yet to effectively translate into a decent average... I'll even see a mid-30 instant if I'm behind a few cars or a large truck, but again... my average is not affected.

Tires: Current setup is 18x7.5 wheel with Yokohama Avid Envigors, 245/45-18. That size is the same overall height as the stock 17" Wheel/tire combo, so there is no effective gearing change. The fuel economy numbers didn't change--on either car--when moving from the stock combo to the 18-inchers. Nor has the type/brand of tire affected the mileage. 2 different sets on the 04--Goodyear something or others and BFG Touring. On my 3rd set (2nd 18 inch set) on the 05--BFG Touring, super-cheapo 18s (Sunew?), Yoko 18s. I have fiddled with pressures to no avail, currently having run from 33psi to 41 psi, with the only difference being a slightly bumpier ride (surprisingly, not awful at 41 psi. I have been VERY pleased with these Yokohamas!).

Again, same mileage circumstances on both cars from the time I got them (~40,000 miles on the 04 when I got it, 22,000 on the 05 when I got it, currently at 98k). Mods, tires, wheels have made no mileage difference. Driving style has negligible effect. MAF has been cleaned. No codes/lights.


My last fill-up was just under 18 mpg... :-\

My GXP regularly averages 20ish with a good amount of highway. I have gone as high as 19 in town and 30 highway...that was some pretty flat road though...

Sounds about right to me... These aren't advanced fuel sipping cars, they're inefficient, heavy, powerful cars designed when fuel was ~$1.00 a gallon. My 99 Regal got around 19 mixed, with a max (road trip) of 28.

-Rob

A 3.5 on a GEN V with only a downpipe and rocker. Sounds like alot of trouble too me.

Those plugs are too cold and that pulley is too small for your "other" mods in my opinion.


Never been a fan of ZZP tunes, its very possible that is a cause to the poor mpgs, along with the plugs.

True... they are not exactly designed to be misers on fuel economy, but I get nowhere near the rated economy. Also, I had a carb'd, 84 Z28 with a 5 speed that would get 31MPG on long trips. To be sure there's been improvement over the subsequent 20 years between production! Especially with a fuel-injected V6 having only 75% of the displacement of the 5.0 (H.O. no less! haha. 195 HP! lol). But, what SHOULD be capable--power and economy-wise--from this 3.8 is not the point. The problem is that it doesn't come near what it is reported to do, factory or otherwise. :-\ Most of the time, I don't even average the estimated city number with my combined 30/70 city/hwy.

The positive side points to the obvious-You gained power, but didn't sacrifice mpg to do it.

The issue lies mostly with in the tune. ZZP's, like other shelf tunes, are purposely tuned to have your car run rich. If you want mog's to increase you will need a custom tune to accomplish the biggest increase.

Holy boost, how much KR does that thing see, I bet its hurting big time.

Try getting your tires Nitro filled, but after all the reading about the PCM. Interested to see how this works out. Heard (not sure) the Nitro fill helps with keeping constant tire pressure which results in fuel savings.

Do you regularly scan for KR? I had to run headers to have no KR

ZZP most likely set PE to come on at ~30-40% TPS to prevent your **** from blowing up, because they know you are risking your engine with that poor mod selection.
Goodbye fuel economy.

Burning any coolant?

Wow. It would be a good idea to put a bigger pulley back on. That's a lot of boost for being non-intercooled.

Originally Posted by AjL227

ZZP most likely set PE to come on at ~30-40% TPS to prevent your **** from blowing up, because they know you are risking your engine with that poor mod selection.
Goodbye fuel economy.

So, PE mode puts it closed loop... working from presets and disregarding 02? Not sure where they have any of that set. I would think that, if it was going to be so dangerous as to require PE at that low TPS, they would have advised as such...??

Bear in mind, before this purchase was made (3.5 instead of 3.6), I asked ZZP IF there was a relatively simple, additional mod I could do to support the 3.5. I was told, by them, that the 1.8 rockers would enable me to run the 3.5. Maybe not ideally, but relatively safely was my impression. This was not done as a "Send me the 3.5 because I want MORE!" I wanted a little more, but not without sufficient support. Had I been told that the 3.5 would NEED headers, heads, cam, more significant mods, etc., I would have stuck with the 3.6. I was told, "You would need a set of high ratio rockers(or 95-96 octane) to be able to safely run a 3.5” pulley without any KR." So, I ordered the rockers believing that to be enough. I'm glad my **** has not blown up! But am now more interested in optimizing what I have.

Also, never had the fuel economy to start with!