I've been considering an enclosed car hauler too, but there's no way to get under 6000 lbs. A 24' trailer will be close to 3000, my 911 track car is 2500, plus all the tools, spares etc adds another 500+.

Right now I tow the 911 on a 950 lb all aluminum full deck open trailer and with tires on the tire rack and the tongue box full of tools etc, it weighs in at 4300 with 500 on the tongue. Pulls extremely well at all speeds but even though I've got the 4.4, there are grades in the Va, WV mountains where I'm donw to 4th gear to maintain 60. Can't imagine adding another ton of weight back there. I'd really like to go enclosed but just don't think there's enough of a safety margin. Towed my old 26' enclosed tagalong with my previous Suburban and it was much better with the 454 and the longer wheel base.

The good news is that I just completed a 1900 mile round trip from Chicago to VIR this past weekend and the X5 performed flawlessly. Even in the WV mountains where I hit heavy snow and 15 degree temps. Averaged 11.5 mpg with speeds ranging from 30 in the snow to 75-ish on the dry flat roads of KY and IN. Did 13 hours straight with only a 15 minute stop for fuel every 225 miles. Got home tired but that's from 3 days on the track, not the drive.

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'04 E53, 4.4 Sport
'97 E39 528i
'86 911 Carrera, track car
'96 BMW R1100R