Ok,
After posting 3 years ago,,, I became a bit OCD about the milage readout. I have experimented and played with the computer in all temps,,, and these are my findings.
My 04 with 3.58 gears will get 30-31mpg at 65-70 in 90 degree temps. This is with the sunroof open, and the drivers rear window down a couple inches. Continental DWS tires aired to 40psi, and 80-100lbs of extras in the trunk. My round trip commute for the last 3 years has been 90-120 miles daily on the highway.
Other findings.
- concrete roads vs. asphalt, will eat about 1mpg.
- AC/defrost will eat 2-3mpg.
- high and low humidity will eat about 1mpg.
- 91 octane will eat about 1-2mpg vs. 93 octane.
For every 10 degree difference in temp,,, the mileage differs by 1 mpg. Yes... I have even tested this down to -20 degrees,,, and it only gets 20mpg when that cold. Keep in mind that I live and commute on flat ground,,, so the biggest hill I see is an overpass. Elevation is about 550-650ft above sea level.
Other factors.
- I seafoam the tank twice a year.
- I am using Bosch Iridium plugs.
- Coils were replaced 70k ago.
- Vehicle has 143k on the clock.
In the end with some city driving,,, the computer ends up reading 25-27mpg average. So I guess the estimated EPA rating of 24mpg is pretty close.