What's the reasonable average speed for trucks in North America?

Not sure why you're excited about that. Your overall average is only 40mph, and that overall average would be practically the same thing, compare to the methodology used by me, Long FLD, AModelCat (which is odometer / engine hours) as well as rollin coal (same method but the data coming from shorter time frame, not the entire lifespan), if it satisfies the following constraints.

• Garmin's 'total time' is exactly the number of hours the device has been powered on (regardless of its status, such as whether the destination is set or not, etc)
• your Garmin is always on when your engine is running, and always off when your engine is not running.

Moving time on Garmin is pretty much useless (it could be useful if everyone is comparing that, since at least everyone is on the same standard, but if you compare it with odometer / engine hours type data, that's comparing apples and oranges), because I (and pretty sure you also) don't know the exact parameter Garmin uses to differentiate between moving and stationary time.

If you ever saw raw GPS data, you'd know that the positioning estimates from each signal refresh NEVER stays on the exact same spot. Rather, it constantly jumps around (usually within 1-50ft circle). In other words, something like the speed you see on your Garmin as well as the position icon on your map, is not exact representation of raw data. It interprets the raw data and creates somewhat imaginary to provide more useful data that is more in line with real world (otherwise, your GPS will show you some speed when you're completely stationary, and your position icon constantly moves around your actual position, and your distance on GPS would be far longer than actual distance).

What all this means? It means GPS is not really accurate at low speed (especially on <5 mph) and also it's not really good at differentiating between slow movement and complete stationary (especially if it's set primarily for vehicle use), hence it probably uses some sort of threshold speed or positioning difference to differentiate between moving and stationary time. If threshold is set to 10mph, then huge portion of your slow driving time (around truck stops, loading/unloading point, etc) wouldn't be included on your moving time and this will boost up your moving avg speed significantly. To determine its accuracy, you have to know its algorithm and parameter exactly, but since that is unknown, it's pretty useless at this point.

If you want to compare apples to apples, then just use the same methodology, odometer / engine hours. Of course, even the exact same driving can result in drastically different data with this method (a person who idles all night vs who stops engine immediately on every stop), and that's why I try to look for someone who never (or at least rarely) idles.

Is this coming from your truck? There is three different time based data shown on the pics (time, drive, sweet spot) and drive is higher than time which doesn't make any sense to me. However, judging from fuel used and avg fuel economy, the trip is likely somewhere around 252,918 mile (6.2 * 40,793.3) and zero chance go over 335,121 mile assuming that's your odometer.

• 27.2 mph = 252,918.46 / 9277.2
• 20.7 mph = 252.918.46 / 12,167.2
• 36.1 mph = 335,121.1 / 9277.2
• 27.5 mph = 335,121.1 / 12,167.2

I guess you idle a lot.