No experience yet...but my race buddy has a 2010 x5d. He recently towed his 3000lb racecar on a uhaul car hauler, 4 spare wheels/tires in the x5 plus tools etc.
He said he was getting 18+mpg doing about 80mph. He said it towed really well.
iirc the uhaul trailer is fairly light.

I tow a similar setup with my 4.6 and get 13mpg doing 70mph.

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Franklin Joseph
11 X5d - 01 325 - 11 1M