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.

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