The 235/75X15 tire is only 15 mm taller which is about .6". That's about a 2 % difference, which is how much lower your speedo will be. It shouldn't affect your mpg enough to notice after you correct for your odometer being 2% lower also.
97 ZJ 5.2, 3.8 Atlas TC, 488s, 8.8 rear w/ARB locker, hp30 w/Oxlocker, Alloys F&R, 35" KM2s, 7" Clayton L A, ARB Bull Bar/Warn, ProtoFab w/ tire carrier, Clayton skid, rock sliders, ORO dual rate Swaybar, Redneck steering assist
97 TJ Juggy, 350 V8, 700R4, D60, 14 bolt, locked, 14" ORI Struts, 113" wb, 42" Pit Bulls, Stazworks beadlocks, F&R winches, caged, Anti-Rock, PSI full hyd steering,