The
FHWA list has the following entries as full Interstates
I-785 North Carolina 2.21 0 - - - 2.21 Mi on I-840
I-840 North Carolina 5.77 2.21 - - - 3.56 Mi on I-73, 2.21 Mi on I-785
The I-73 mileage isn't on the main routes table, AFAICS, but both Fut I-840 segments are unsigned I-840, the west one also unsigned I-73 (but I'm not sure the other bit of what we have as Fut I-73 is) and the east one also unsigned I-785.