Across the western US today the freeway default posted limit is 75 in these states:
1)Texas as of Sept. 1, 2011,
2)Louisiana as of April 11, 2011 on I-49,
3)Oklahoma,
4)Kansas as of July 1, 2011,
5)Nebraska,
6)North & 7)South Dakota,
8)New Mexico,
9)Colorado,
10)Wyoming,
11)Montana,
12)Arizona,
13)Nevada,
14)Utah,
15)Idaho
All of these states have 75 as their default speed limit on rural freeways, 65-70 on rural 2 lane highways. Two states, Texas and Utah now have stretches posted at 80 mph today. Maine is considering joining the 75 mph club. 35 states today have at least 70 as their posted maximum today. With allowed speeds across the US today of at least 80-85 mph on most rural freeway stretches and 65-75 mph on rural two lane roads.
In the Denver metro area freeways are posted 65-75 today. Many US cities have 65 posted for urban freeways. Across CA the urban freeway default limit is 65 mph with average travel speeds of 80 mph. In CA anything short of the high 80s to 90 mph on most rural freeways will not get a CHP officers attention.
So in the real world across the US today 60 mph is impractical and dangerous for you if you are driving that slow and to those around you on any freeway and most rural highways across the US. In the west we often set the cruise for hours on end @ 83-85 mph. So that is what I use as a real world speed today. And at that speed I bet this thing will have a hard time getting out of the high 30s mpgUS.