I've seen 22-23mpg on pure highway, esp. when cruise control is used. 2 things I've observed which should be obvious but I'll point out anyway:

Mpg begins to drop as your speed exceeds 80mph. My avg. mpg was consistently climbing until I exceed 80+, and begins to decline until I'm <80 again.

Keep that mpg needle above the 20mpg mark, which is sometimes difficult to do unless cruise ctrl is used. I use the cc stalk to adjust speed instead of gas/brake/reset speed.

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11' X5 35i Sport Activity - AW/Blk/Alum
06' Subaru Legacy GT - GONE
05' BMW X3 3.0 - GONE
01' Honda Prelude, modded - GONE