Really Drone, 6-10% My 35's only throw the speedo of 5mph |removethispart|@ 50mph... (I'm actually running 55.) Thats 10% for a tire that is hugh in size difference compared to a 235. I am running stock gears for now too.

dangit...
My gas mileage is pretty decent although I have never checked it for an actual figure... (it is a 1.6 8V though)
I would think he wouldn't notice much difference in any area except maybe the bumper.
I know I didn't when I put 235's under my Vitara....