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... :-\
