I'm sure there were probably several varibales that affected the change. As far as speed, once I left Oregon (about 3-3.5 hours of the trip) the speed limit was 75 mph the entire way, except through metropolitan areas. My first refueling point was in Mountain Home, Idaho, where the octane level changed from 87 to 85 for regular grade gas. The fastest rise in elevation occurred during the first couple of hours of the trip, going from 500 feet to 3000. However, the elevation continued to rise albeit more gradually. As I've been in Colorado for a week, I will have to check the computer to see if I have had an effective change in mpg for 'in and around town' driving between the two locations.