Maybe not
I think that there is some wiggle room here, guys. I went from the stock 205's (getting about 18 on the highway) to 235's (29.5") and got better mileage with 3.73 gears.
I ran Pirelli's for years, and now I have Michellin's, and I get 20-21 mpg corrected.
Now I'm sure you could put a lot heavier tire of the same size, or one that doesn't roll as well, and drop mileage.
I'm also sure that if I went to any taller tire with those gears, I'd have a drop in mileage, too.
-zz