Towing with e70 x5d: 20" vs. 19" vs. 18" Wheels
 

I have the Sport Package Style 214 20" Wheels on my e70 LCI x5d and was wondering if getting the smaller 19" or even 18" OE wheels just for towing would improve anything (mpg, ride quality, tire wear due to rear camber, etc) while towing?

Maybe ride quality...but how much towing are you planning to do? Wouldn't think it's worth it. Mine is just fine with 20's.

Maybe once a month 100-400 miles.
Since used OE 18" & 19" wheel/tire takeoffs are relatively affordable, makes me wonder if its worth it... to save on the expensive 20" run flat tires and if it improves towing mpg.

I highly doubt switching from 20 to 18 will improve MPG when towing... Wind resistance of whatever you tow probably has greater impact

Tire diameter isn't going to affect mpg much at these sizes when towing.

For towing, you need to consider a few key factors:

1) Lower profile sidewalls are generally better, because they provide more lateral stability and can reduce trailer sway. Tall sidewalls don't do as much to reduce sway. All of these are relatively low profile tires, however.

2) Some larger diameter performance tires will have lower load ratings. Load ratings matter when towing, as tow vehicles are sometimes at maximum rated axle loads, depending on the weight of the trailer.

3) Cost and reliability/durabilty. Flats are more of a hassle when towing, and can put you in a dangerous situation. Lower profile tires cost more, wear faster, and are more prone to road damage (punctures). All of that can get amplified at higher loads.

I would go larger due to (1), within the limits of (2). I thought for awhile a month ago about towing with a new E70, and if I had gone ahead with it, I would have purchased 19" from new. That doesn't mean it is worth changing from 20", though, unless (3) is a significant factor.

All good points, especially the punctures while towing.
I guess I will stick with the 20" Dunlop Runflats that came with the car and see how that goes. I think they would be considered "low-profile" @ 275/40/20 and 315/35/20 ... and the load rating for the 315/35/20 Dunlop RFTs is 2330lbs.
So weight of the car in the back, say 2,600-3,000 lbs. + the max. 600 lbs. tongue weight = 3,600 lbs. spread across both tires leaves me with 1,000 lbs. extra load across the 2 tires.

Running RFTs would only solve the danger aspect of a puncture, but you would still have to get a $500 replacement RFT tire wherever you happen to be. But you have 50 miles or so to get to the tire shop with your rig (or will a RFT not allow you to run flat while towing??), and I suppose thats better then waiting for a tow truck on the side of the hwy.

What I'm worried about now is the additional forced camber on the rear tires from the tongue weight. I wonder what that additional neg camber is.

I knew a guy with an x5d who towed with 22" wheels and the inside wear on his tires was to the cord while the outside still had 60% left. Trying to avoid that.

How much are you towing? If it is over 5000 lbs, think about a weight distributing hitch.

22" is just silly, IMO. And most inner tire wear is more related to the toe-in setting on the rear, than the camber.

Open trailer = 1,900 lbs (approx.)
Car = 2,400 lbs.
Wheels/Tires/track stuff = 200-300 lbs.
So approx. 4,600 max

110% agree on toe. Nothing wrong with camber, but even mild toe will accelerate wear dramatically with a cambered wheel....


Do they recommend a weight distribution hitch on the X5? I assumed the mounting points were not designed to handle the moment which is transferred to the frame....

BMW don't recommend weight distributing hitches, but that has nothing to do with a WD hitch, IMO. They aren't legal in Europe, and BMW designed the vehicle in Europe, so they don't recommend them. BMW (and other manufacturers) don't understand them, don't know how they will be used due to the variability of each user applying a different WD load each time they hitch up, and so just stay away from them. More to do with legal than good practice. If I was towing a significant load, I would be using a WD hitch, no questions.

The hitch tongue load puts a bending moment on the hitch receiver in normal operation, at the interface to the vehcle (where the receiver is mounted). A WD hitch applies an opposing bending moment, so the bending load isn't cumulative. If a user applied sufficient WD to load the front axle much beyond its empty (non-towing) weight, then that could be a significant load transfer. All things in moderation.