Today I tried as scientific as possible of a highway efficiency test. I made sure all four pressures were 42 psi cold, per the sticker in the driver's door. I removed the front plate holder (PA has no front plates) and installed a customized euro plate I've had for years from other cars. On a limited access toll highway with a 70mph limit, with about 500lb of people and gear in the car, I accelerated as quickly as possible to 80mph, set the Travel Assist adaptive cruise, reset the short term trip odometer and cruised 12 miles to the first exit, got off, turned around, and repeated the process in reverse.

Start Elevation: 225 ft
Outside temp 58-59*f
265 miles of range showing
82% battery capacity
HVAC on 65*F, no seat/wheel heaters
12 miles
9 minutes
79 mph average (a tiny bit of traffic, especially at the end)
2.1 mi/kWh
End Elevation: 500 ft
226 miles of range showing
75% battery capacity

-got off at the exit and back on-

Start Elevation: 500 ft
Outside temp 58-59*f
224 miles of range showing
73% battery capacity
12 miles
9 minutes
79 mph average (again a tiny bit of traffic, especially at the end)
2.9 mi/kWh
End Elevation: 225 ft
205 miles of range showing at the end
68% battery capacity

Pardon the glare (the white trim does no favors) and greasy fingerprints but here is the result at the end of the first leg driving North:


Here's a photo of the end of the trip back where I started:


Average efficiency was 2.5 mi/kWh as shown in the title of this thread. I also logged some data in the 8C-Hybrid Battery module using VCDS. I was surprised by how fast the motor turns at 80mph (12,400rpm !?!) but I remember reading that the APP 310 motor used in the ID.4 has a limit of 16,000 rpm so this makes sense, extrapolated out to about 15,500 rpm at the governed 100mph.

Here are some more graphs from the data, and please let me know if anyone wants the raw data (10's of thousands of lines):