Here's my latest version of MPG recommendations:
*upshift @ or below 3.3K RPM... early & often... I upshift as low as 2K RPM quite frequently. There are two GT-S RPM sweetspots for mpg 2.3K & 3.15K. It doesn't really lug till ya get down close to 1.2K RPM. Downshift late, 1.5k is about my usual limit at a steady speed (upshifting is different). 6th gear is the most efficient (slight overdrive)
*clean the Throttle Body/Intake manifold & IACV (your method of choice.. I just use carb cleaner)... every 15K. Seafoam will help higher mileage or city driven cars with cleaning tops of pistons.
*Air filter cleaned every 15K (replaced every 15K, if stock)
*MAF cleaned every 15K. CRC's MAF Sensor Cleaner preferred. If you have a SRI or CAI, verify your MAF reading @ idle to ensure that you don't need to BBmod your intake.
*oil/filter change every 2.5K miles (oil analysis wear #'s are very low & they predict a 200K+ engine life, I'm aiming for 300K) From a crude extrapolation of my oil analyses, I don't recommend an oil change longer than 3,750 miles.
*use an API Energy Conserving 5W30 Oil (unless you are in a higher or lower than normal climate)... a synthetic, if you can afford it. If not, consider a less expensive synthetic Blend. Try to stay w/one brand... while oil is oil... the additives & their quantities vary by manufacturer. My oil of choice is now Castrol Edge. It has done an excellent job @ keeping my engine clean & preventing wear.
*18" rims will hurt your economy... I'm using the standard +1 16" OEMS. Might use low rolling coefficient tires. Bigger rims are often heavier and that rotational mass will have an impact on your mpg.
*add fuel injector cleaner every 2.5K miles (after every oil change) I use the basic Walmart Tech brand.
*Redline MT90 tranny oil change (I had been doing every 15K.. but, I've lengthened to a more common 30K.)
*Mobil1 Synthetic transmission fluid in Power Steering. I drain & fill the reservoir @ ea. oil change. Not that it should impact mpg... I do use a Gatorback drive belt which has helped steering lag from belt slippage considerably.
*Use the undercarriage plastics or get a shield to reduce drag, improve engine cooling, and keep engine bay cleaner.
*Check tire inflation
@ correct pressure (min. once a month). I go 2-8psi over for highway driving in Spring, Summer, & Fall. Placard pressure or slightly below in Winter. FYI, plus sized rims & tires usually need 0-3psi over placard.
*Proper wheel alignment done at least annually
*Rotate tires on a regular basis (I do every 5K)
*Use cruise control
(it is usually better @ managing throttle than a human & a steady speed is more economical)
*The lower in the fuel tank you go, the better.
When the bars go totally away... there is usually somewhere around 0.5-0.75gals fuel left, plenty for you to find a station. If you are currently refueling @ 2 bars.. go to refueling @ 1 bar (wait for the fuel light), unless you know you will need more fuel for your next trip. A fully loaded fuel tank adds almost 90lbs or so. If you aren't tracking MPG, aren't travelling a long distance,... or have limited funds...you can probably benefit by only filling to 1/2 tank.
*No junk in the trunk
... the lighter, the better. I can see differences in MPG just carrying an extra 50lbs. Load placement also helps. I see better economy putting extra weight in the passenger seat than in the hatch area.
*a clean car doesn't hurt... waxed... even better (slight drag reduction)
*minimize fan & electrical usage. In the winter... try turning up the heat higher & turning off the fan (unless you are sitting still). This acomplishes a couple of things... the radiator fan will not run as often and you aren't using the heater fan.
*on the highway.. A/C might be more efficient than an open window (increases wind drag). If using A/C, set it all the way cold & only use as much fan as needed. On moderate days, leave windows closed & use fan on 1st or 2nd setting vs. an open window. If you must open a window (for smokers)cracking the sun/moonroof seems to work better/less drag than cracking a side window. NOTE: Thus far, my ScanGauge MPG readings don't support the A/C being more efficient theory. However, sometimes driver comfort is important. From experience, A/C use does seem more efficient on 6cyl. vehicles where it is less of an additional load than on a 4 cyl. I only take a 1-2 MPG highway loss on my Avalon, compared to a 2-4 MPG loss on the Celica, using the A/C.
putting the HVAC in either of the two Defrost modes.
The AC compressor runs in those modes.
*I use the stock OEM long life platinum Iridium plugs... replaced every 60K (I may try the IK's next time... to see if they help MPG @ all). Check gap (carefully). DENSO has come out w/a new Long Life Performance Plug the VK (Iridium Tough). I haven't tried it... but, it looks promising. I haven't tried the IKs.. but, they may provide a minimal mpg improvement.
*Use a top tier fuel
(I use Shell 93 V-Power) Shell has long been known for better fuel economy. Chevron may provide better performance w/less economy... I'm not sure. A GT may see better MPGs w/89 Octane. Even if your using 87, I recommend a top tier fuel for a GT.
(if you're @ a long light & back in the pack.... shut off your engine. I've stopped doing this due to possible wear on the VVT locking pin.
*Avoid jackrabbit starts & screeching stops.
* Avoid short trips.
Plan your trips, go to the furthest destination first and then make the rest of the stops on the return. This will insure a warmed up engine and better economy.
*Coast whenever possible
.... down hills... come out of gear. Come off the accelerator when a light changes to yellow (no sense hurrying to a redlight... & it may change by the time you get there - reducing idling). Take advantage of DFCO (Decel Fuel Cut Off = 0 fuel use, the tranny pushes the engine) by coming off the gas & stay in gear to slow down for off ramps & traffic lights. It cuts fuel use and saves on brakes (Not to be confused with downshifting for engine braking). You have to balance the use of DFCO vs. the distance you need to cover. A combination of DFCO to slow and coasting to reach the end of a segment is what I use on offramps. Stoplights are often DFCO only, depending on the visible distance to see a red light or a "stale" green. Try to gauge your coasting to minimize idling at the light. Often, the light will turn green before I reach it.
*keep your speed in check.
*use a light right foot
... preferably under 1/4 throttle.... to as little as 5-10% throttle... I know it seems slow... but, if you upshift etc. you'd be surprised @ how easily you keep up with traffic ahead of you & how soon you make it into 6th gear.
*Optimum superhighway speed for a GT-S appears to be around 3120-3150 RPM/68 MPH. I seem to get better mpg @ 68MPH than I do @ 65MPH. From my testing, anything over 70MPH is going to reduce your mpg. There is another RPM sweetspot at around 2.3K. If you are below highway speeds, this is a good RPM to run in any gear. These sweetspots coincide w/torque dips on most GT-S dynos. Torque dips on a GT appear to occur around 3K and 3.8K-4K. The second one covers 68-78 MPH for a GT. So, I suggest trying 68-69MPH (3.8-3.85 RPM) for a GT as well.
Advanced techniques include Pulse & Glide, Driving w/Buffers, Driving w/o Brakes, Driving w/Load, drafting, engine off coasting, and more.
Driving style & proper maintenance are the biggest factors.
This is a pretty good article: http://www.washingtonpost.com/wp-dyn/content/article/2008/06/14/AR2008061400127.html
that helps explain some of the mental changes you need to make.
Keep in mind that, while I qualify as an "expert hypermiler", I am by no means as extreme as some.