My truck is an SRW, not a dually.
The data was over different routes. However, I made sure the starting and stopping elevation was always within 20 feet. Also, trans temp was always between 136 and 144. Finally, wind was typically at a minimum. Despite those relatively constant variables, the variation in the data was still pretty large. That's why I kept getting more data points. I will add that I found no difference in MPG as the outside temp changed. In fact, there was a slight downward trend with outside temp when I plotted the numbers for 65 and 70 mph.