I've regularly done the 1100 mile trip between Coppell, TX and Spring Hill FL with one gas/restroom stop, 60 miles to E showing on the OBC upon arrival. Actual calculated MPG of over 24 (GPS miles and gals pumped). OBC can be inaccurate.

This is done with the 255-18s all around (my hwy/towing tires, 38 psi pressure), under 75 MPH, following 2 seconds behind 18 wheelers when possible, no AC and windows/sunroof closed.

Stop and go around town I see around 420 miles per tank.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!