What will the type of tires do to my gas milage on my jeep wrangler?

I have 235-75-15 tires now on my jeep with factory rims, If I put aftermarket rims 15 x 8 on my jeep what would my gas milage do if I went up to a 30 x 9.5 x 15 tire size. If anyone has these tire sizes with aggressive tread and all terrain tread and has them on a jeep wrangler and could let me know what the gas milage is I would appreciate it alot.