So I've been collecting data since my last post, recording the odometer reading, idle hours reading, and drive hours reading about once every two weeks or so. Here is what I've concluded:
(1) The Blue dots are the idle hours. The current value is 325 hours at 56,022 miles.
(2) The Orange dots are the drive hours. The current value is 515 hours at 56,022 miles.
(3) Notice since I began recording the blue and orange dots, they have maintained a highly linear rate of accumulation. Adding trendlines to both sets of data (orange drive hours and blue idle hours), I get the slope of each, which you can see on the associated linear equation.
(4) Assumption: when the vehicle was "born," both the idle and drive hours were zero (0).
(5) Assumption: Since the vehicle was "born," both idle and drive hours have been accumulating at this same rate (slope). I know this is a pretty big assumption, but I have no way of knowing what the driving habits of the only prior owner was, and I have been driving the truck since I bought it at 1.5 years old and around 17,600 miles, 70% of the vehicles life. So I'm going to make this assumption because it probably isn't that far off.
(6) I've graphed back-projected lines (yellow for idle hours, gray for drive hours), setting their slopes the same as the recorded data trend lines, but setting their y-intercepts to zero, simulating a brand "new-born" truck.
(7) You can see that the back-projected idle hours line (yellow dots) comes in about 60 hours less than my current value. However, idle hours also have the ability to increase on their own without a corresponding increase in the odometer reading, since you can be idling while sitting still. So, I actually believe my current idle hours value is correct: 325 hours.
(8) Drive hours do not have the ability to increase without a corresponding increase in odometer reading, so that means that the back-projected drive hours line (gray dots) is the more likely true value. As you can see, it goes off the chart. If you take it's reading at my current mileage of 56,022 miles, it is 1,910. Way more than 515.
In conclusion, at some point the drive hours reset back to zero around 41,000 miles, causing it to have to start over again. I have no idea why this would happen. But, I think my logic above makes sense, so I'm going to go with it. That means my current ratio of idle/total hours is 325/(325+1910) = 15%. Much better!