Welcome to CleanMPG!
Your 03 Envoy, if it's a 4x4, is EPA rated at 15/17/21 (city/combined/highway) under the pre-08 rules, so you're already "above EPA" on that 23.4mpg tank. Congratulations on an excellent start!
About five weeks ago, I ended up having to drive an '04 Mountaineer for a few hundred miles. EPA 15/17/20, or about the same as your Envoy. Tires were already set at 42psi, so I didn't see the need to boost them much higher. But this Mountaineer has a problem in the AWD system - either transfer case or hubs - as it'd "lock up" when turning hard, and it'd drag down during coasting as if the e-brake was on. The owners of this beast get 14mpg highway! When I was handed the keys to drive it home and park it while they were off in Europe, I noted the average FE display was indeed 14.1 after 180 miles of highway driving. I reset the display and drove it. Ended up at 21.2mpg, which is a 50% improvement over what the owners average.
- Easy acceleration with momentary releases of the throttle to encourage upshifting.
- DWL ("Driving with constant engine load") - Let the car lose speed uphill and gain speed downhill, and keep the engine loaded up and almost lugging.
- Speed capped at 55mph, since these SUVs have a high aero drag coefficient and lots of frontal area and so 60+mph will send your gas gage diving.
- While moving, avoid the brake pedal as much as possible. Coast down when you need to lose speed.
- In effect, drive as if you were driving on icy roads. Smooth gentle driving and lower than normal speeds.
If the Mountaineer was in good working order, I would think I would have reached the high 20s in it, considering that the average for it is 14.1 and that's about 25% below what you'd expect of it. Add 25% to my 21.2mpg and you get 26.5mpg.
The silver lining to my accomplishment is that the owners saw the mpg number when they returned, and marvelled at the major improvement. When I told them that they likely had a problem with the AWD and that is the reason for the poor mileage, they actually listened. They said they thought something was wrong with it, but the dealer said it was ok. (Dealers always say it's ok, until the warranty runs out...) So now they will get it repaired and save themselves a lot of gas money as well as saving our country a lot of money sent half way around the world!
Keep up the good work with your Envoy. This is what CleanMPG is all about - saving fuel with simple changes in driving routines, and sharing the knowledge with others.
Rather than trading out of your SUV and losing thousands of dollars and seeing it sold to someone who will beat on it and get 10mpg, you can hypermile it and boost fuel economy into the mid to high 20s. And that's about what Joe Punchclock gets in his Toyonda Corollivic as he races down the interstate at 20 over the limit, jamming the brakes every half mile. It's not what you drive, it's how you drive it.