The load setting (or LOD) is the load on the engine. The higher the load, the more fuel being used. That being said, you sometimes need a lot of load like pulling a hill or getting up to speed from a stop. This number should follow the same pattern as your instantaneous MPG though when you change speeds your MPG will rise higher whereas your load will always have a set top and bottom. I guess technically the numbers will be inverse of one another. high load means low MPG and low load means high MPG and that trend should hold true during a given gear.
I would just use instantaneous MPG and trip MPG for mine (and do). Generally speaking, the less load on the engine, the better your MPG will be. there are exceptions to that rule like getting back up to speed and using pulse and glide. I actually used this when trying to see if my headlights would make a differenc (on or off) and it showed a noticeable difference in load. it has been a while since that experiment and I can't remember the exact numbers but that was about the extent of the usefulness of that parameter for me (LOD that is).
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I'm assuming that 5 speed is a manual? I think your vehicle also has cylinder deactivation. If so, you have a lot of potential in that vehicle. Keep the speed as low as you can in 5th gear without it lugging, and you will be amazed at the mileage you can pull out of that.
Higher load is a good thing, as long as you're not wasting the energy produced. Search the forum and the internet for BSFC (Brake Specific Fuel Consumption), a measurement of an engine's fuel efficiency (not a vehicle, just an engine).
In practical application, this means that while you don't look to maximize your LOD statistic, you do want to make as much torque as you can use, at as low an RPM as agrees with your engine. Keep your RPM down and don't be afraid to be heavy on the accelerator. Set one of your ScanGauge's displays to Open/Closed Loop and avoid Open Loop, which you may get at WOT (Wide Open Throttle).
That was a major part of how I got great results from my 2008 VW. That car was also a very good match for that strategy, though; it had too much torque and ran smoothly right from idle RPM even at WOT. It also had a wideband O2 sensor that allowed it to stay in closed loop at WOT. The 2007 Sierra 4.3 probably doesn't have a wideband O2. However, to its advantage, I bet it has decent wide ratio gearing so you can keep RPM low at a variety of speeds...that VW had a gear for every speed, and that gear was 5th. I could drive that car only ever using 2nd and 5th and it would be like using all 5 gears in my Buick.