IMO....there's already soooo many things thats already going to damage your mileage a little.I mean if you never plan on...driving against the wind,locking it in on bad or snowy roads,a little towing from time to time,wheeling a trail,or just accelerating a little hard now and then.....you might consider keeping your stock wheels.Oh yeh...keep a good coat of wax on her,cause I'm sure with some testing it would cause less wind drag.I mean I hate to be pestamistic here, but for all of the things that your libby is capable of,it's worth another gallon or two of fuel.So my guess would be yes it will use a little more fuel....but I'd bet no more than 1 to 2 miles per gallon.Sorry to rant,but this is one sore topic for me.Thats why I bought mine,to do all that it's capable of.
Ron