I expect the 777s and 339s will be assigned to maximize utilization of these very small subfleets. 339s will have low trip costs/lower per seat fuel costs but the two types will have substantially the same number of seats and range. There are only eight 777-200ERs, with an average of ~19 years. Delta won't retire them early because they have more pressing replacement needs. Even after receipt of ~175 739s and A321s over the last few years the average age of the DL mainline fleet is 16.2 years (AA, 10.6; WN, 10.9; UA, 15.0). The 339 fleet will be few until 2022 or later. Once DL has enough 339s to stop worrying about utilization and focus on fuel savings, look for present 777 routes of 6,000sm or more.