I thought a taller tire would help reduce fuel consumption on the highway?

yes you are right TALL tires reduce fuel mileage

Wait you just contradicted what I just said.. tall tires should HELP fuel mileage because the motor is turning less rotations per minute at the same exact speed as opposed to having tiny tires..

ya i just reread. the taller tire may help fuel mileage if your just driving but having the tires the proper size helps when towing. also if you run 235s watch your speed they will throw it off some

Having the taller tire on the left inner is going to put most of the load on that tire and cause the outer tire to spin a little bit faster, depending on the actually overall difference. If theres a BIG difference it might wear the smaller tire faster than the rest. As HAYMAFIA mentioned the 245/75R16 is really close in height to the 215/85R16 that came stock. Biggest difference is the 245 is more available and a touch wider. But as far as the 235/85R16 rubbing in the front i have never heard of. 99% of the Dually's i see are running the 235/85R16 all around. Hope this helps.

mine always rubbed the inner plastic. i just trimmed them or took them out completely. this is on a 2 wheel drive

you shouldnt have to worry about your speddo unless you go up a couple tire sizes, i ran 265-75-16 and 285-70-17 and the speddo was only off like one or 2 at 70 mph,
but now i have 285-75-16 and its off little bit i can tell, im gunna check it out this summer when they put those speddo things up on the side of the road